[TinyTim]

Hey Ice, I have the Yak-9U pilot manual, but there's no mention of the UT version in it. Turning over to second hand - there's probably no author I'd trust more on Russian world war 2 aviation than Yefim Gordon.

In his excellent book "Soviet combat aircraft of the Second World War - Volume 1: Single engined fighters" this is what he has to say on Yak-9UT armament:

Yak-9UT
The Yak-9U and the other Yak variants were used to test a range of armament alternatives; 23mm, 37mm and even 45mm guns. To install the last of these it was necessary to remove the synchronised machine guns, B-20 synchronised guns being installed instead. The possibility of installing different enginemounted guns without the need for airframe modification was the design's most valuable feature, allowing a rapid change of armament, according to WS requirements, during series-production. The aircraft was designated Yak-9UT. Clearly, each armament installation a different effect upon weight, centre of gravity position and flying performance, but the speed was the same as the Yak-9U. Handling was almost the same except for elevator load, which was heavy, this being the aircraft's most serious shortcoming. Main advantage of the Yak-9UT was its heavy salvo weight, 13.2Ib/sec (6kg/sec), using the NS-37 and two B-20s. This was a formidable figure at the end of the Second World War, even for the Germans, who were trying to increase the salvo weight of their aircraft as much as possible for combat with the redoubtable Boeing B-17 Flying Fortress. Testing of the Yak-9UT began in March 1945 and was completed, very successfully, after the war's end, but many of the 282 built took part in air combats over Berlin during the last days of the war.

Another great resource from him is "Yakovlev's piston engined fighters", this is a Yak-9UT section:

Yak-9U versions with reinforced armament

Like many other basic Subtypes of the Yaks, the Yak-9U served as a kind of proving ground for developing different armament options. In one machine provision was made for replacing the ShVAK-20 cannon in factory conditions by more potent cannons - an NS-23, an N-37 or even an N-45; in the latter case one of the two synchronised weapons in the front upper decking had to be deleted to lighten the machine. Synchronised machineguns were replaced by new B-20 cannons (synchronised, of course).

The possibility of installing different engine-mounted cannons without any major redesign of the aircraft was a distinct advantage, making it possible to switch series production quickly to this or that type of armament, depending on the requirements of the Air Force. The NS-37 and its derivative, a 45-mm cannon, were experimental lightened weapons developed by OKB-16. A prototype fighter featuring this kind of armament (c/n 39166083, ie, Batch 39, Omsk aircraft factory No. 166, 83rd aircraft in the batch) was built under A. Yakovlev's direct guidance in February and allocated the designation Yak-9UT, the T denoting tahnkovyy (tank-busting).

Naturally, each armament version had its own all-up weight, CG position and flight performance. But the speeds were identical to those of the production Yak-9U. The handling qualities of the Yak-9UT were virtually the same as those of its predecessors, except for the control stick forces from the elevator: they proved to be too high, and that was the most serious shortcoming of the aircraft.

On the credit side was the weight of fire: with the installation comprising one NS-37 and two B-20s, it amounted to 6.0 kg/sec (13,2 Ib/sec) as compared to the Yak-9U's 2.81 kg/sec (6.2 Ib/sec). In the final stages of the war such sizeable figures commanded respect even from the Germans who also strove to increase the firepower as much as possible in order to fight the sturdy and highly survivable Boeing B-17 Flying Fortresses and Consolidated B-24 Liberators. Tests of this aircraft were conducted by engineer G. A. Sedov and pilot A. Manucharov at Nil WS in March 1945. The Yak-9UT proved to be considerably more stable under different manoeuvres compared to the Yak-9T and Yak-9K, owing primarily to lesser recoil of the cannon and greater speed envelope. The aircraft was recommended for series production, and Plant No. 166 delivered 282 machines with the engine-mounted NS-23 cannon and two synchronised B-20S cannon. An important stage in the history of the VK-1 07A-powered fighters was the emergence of the 'etalon' (production standardsetter) for 1945. The main tasks facing the Yakovlev OKB were to eliminate the main defects of the Yak-9 VK-1 07A; to ensure a top speed of 600 km/h (373 mph) at sea level and 700 km/h (435 mph) at the second rated altitude with the engine at the maximum rating; to ensure a climb to 5,000 m (16,400 ft) within 4.1 minutes and a service ceiling of 11 ,000 m (36,080 ft); to achieve a cruising range of 900 km (559 miles) and a range of 1,200 km (746 miles) in optimum cruise mode. The aircraft was built, using a production Omsk-built airframe (c/n 41166038 ). An important feature of the machine was its ability to accept alternative versions of armament in a manner similar to the Yak-9UT. The fighter was tested with three B-20s (two of them synchronised) and an all-up weight of 3,145 kg (6,935 Ib). Nil WS specialists noted the advantages of the alternative armament options, the improved external finish and pointed out the need for using an antenna mast. At the same time they stated that only some of the Yak-9U's shortcomings had been eliminated. Thus, the installation of a bigger oil cooler with greater frontal area and introduction of an additional oil pump led to better engine running at nominal revs only at high altitudes. As before, considerable oil spill from the breather occurred at maximum power. In the course of 45 flights performed by the fighter, four engine changes (!) had to be made. In the last engine (No.527-21), which was part of an 'improved batch', the crankshaft main bearings broke down during the 14th hour of engine running. An extremely disappointed Aleksandr Yakovlev ordered the aircraft with c/n 41-038 to be withdrawn from testing and returned to the OKB so that development work could be resumed. All subsequent improvements introduced into the design came after the end of the war.

Bottom line - according to Yefim Gordon, all production Yak-9UTs were armed with NS-23 firing through the spinner and two 20mm Berezin cannons in the cowling. Prototype tho appears to have been equipped with a NS-37 (and two B-20s). Considering the way the Yak-9UT was layed out, maybe an armament option of 23mm, 37mm or 45mm weapon should be available to player to choose from. Diversity and adaptability of its armament was the very reason for deriving Yak-9UT version from basic Yak-9U in the first place.