And Milo, even if that drawing says MG 151/20 I think they must have gotten sloppy and drawn the gun AND the propeller shaft/blast tube. The weapon wasn't nearly that long.
MG151/20 installation in 109F-4:


MG151/20 Gondola

MG151/20 wingroot guns in Fw 190D-9:

MG151/15 and /20:

And finally ...BlauKreuz holding a MG 151/20 at the Hallinportti Aviation Museum in Finland:

Fitting a MG 151/20 or two in the cowl of a 109 doesn't look that far fetched after all.