Simon
Based upon my experience with our 3 VIICs-VIIC/41 I would imagine your had major changes in the uboats delivered from the yards as from July 1943. U 995 our oldest VIIC/41 delivered July 1943 and the two other delivered end 1943 had nonreversible engines, removed purifier etc. That would not eliminate possible earlier changes. Installation of the schnorchel was carried out later, for U 995 it was just completed at the end of the war May 1945.
Tore