There were at least two other projected versions of the MK 108, neither of which saw service.
One was the MK 108A, or nA (presumably for "neuer Art" or new type) in which the RoF was boosted to 900 rpm. It was tested but too late for service.
The other was the MK 108 V-5, which was a simplified design (hard to imagine!) to make production even cheaper. This never left the drawing board.
The MG 151 could not fit into the Bf 109 cowling; whether it was the 15mm or 20mm version made no difference. The 15mm gun is often confused with the (much smaller) 13mm MG 131 (just as the MK 103 is confused with the MK 108 - very different!).
Tony Williams
Author: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website:
http://www.quarry.nildram.co.ukMilitary gun and ammunition discussion forum:
http://forums.delphiforums.com/autogun/messages/