Auto-Pitch control.

[Kurfurst]

The first 109Es (production started end 193 came without constant speed units.
The pilot manually adjusted propeller blade angle constantly at any angle, to match engine RPM and torque.

There were two systems for this: the first one, which we have on our early E-3, was a lever on the dashboard, and ideally suited pilots born with three hands.

On the second system had the propellor blade angle set by a thumb switch located on the throttle. This one become standard for the rest of the war.

Finally, automatic propellor govenor system was introduced:

"The constant speed airscrew is fully automatic and requires no operationby the pilot, as r.p.m. and pitch setting are governed by the throttle opening and engine load. This is a great advantage in combat, being one thing less for the pilot to worry about. He can, however, control the pitch manually in the event of failure of the automatic control."

This automatic system appears to be introduced around late 1939, or perhaps earlier; what is known that the December 1939 already describes it, along with aircraft armor. Evidently not all older production Emils were retrofitted by the BoB started, so they had manual control (like our E-3) though it was eventually retrofitted to older Emils.

Since production of the E-4 (the most important type during the BoB) commenced in May 1940, so I suppose when the E-4 will be introduced it will feature the auto prop pitch and armor.