[Buster_Dee]

As said, the Lib I was a B17C. From O'Leary's Production Line to Front Line (paraphrased):

The bathtup belly position with single 50 cal, and the flat waist windows for the waist gunners, confirm this as a C/Lib I. At 1st, I think, there were five 30 cals (nose, belly tub, RO on spine, and two waists--the simlar D had twin guns in tub and RO space), but Boeing quickly upgraded to 50 cals in RO and belly tub. The nose remained a 30, and I don't know if the waists were also still 30s. Where the C's development was when the British took them on, I don't know. Probably safe to say the Lib I had five guns.

I'm guessing the British did not substitute 30s (or 303s) for the 50s, since it is said they were "suprised" that some of the guns froze at altitude. I think they were quite familiar with the 30s (and their own 303 variant) and would have know its quircks. I think they would have been less familiar with the 50's quirks/heating needs.