in the Haynes book, it's stated that there were just a little more than 2000 Fireflys built in all various forms. Thinking on this, one almost has to wonder why? The hulls were 95% standard Sherman issue. Seems that I read once that like the U.S., the Brits modified the 17 pounder gun mount to fit the U.S. style (U.S. did a similar thing with the 76mm mount). So it appears that the rear cut out was the biggest mod done. Yes the ammo storage probably was changed along with all radios and such.
With the above out of the way; why did the Brits even bother to do the cut out in the turret? Same can be said about ammo stowage. Would not have been a little more feasible to cut the rear port out while going thru the machining process. Looks like internal hull mods could have been done in a fraction of the time if they'd been done on the assembly line.
Still one has to assume that the tanks sent to England were ear marked for modification before they went out the door over here. The turrets were a high alloy grade of cast steel. Takes some pretty good equipment to machine it. (I have cut a lot of armor plate) It will flame cut just fine (1.5" or less in thickness). Welds just fine, but also cause some bad issues after the weld cools down. Using a torch to cut the pocket is close to being stupid, but it'll cut just fine. Machining the window is the correct method to do without destroying the metallurgical properties. Cutting the pocket out would have taken about three to four hours with tooling and equipment of that era. (very large and expensive equipment). The ammo stowage and other items needed could have been prefabbed on either side of the pond.
What I see is a tank being built, and then being ripped apart while it's still brand new to make something different. No wonder they only made 2000 of them, when they could have made 8,000 of them! Sure looks like it would've been easier an cheaper to send basic hulls with no turrets, and turrets ready to assemble in whatever configuration that was the flavor of the day.
Not trying to be a troll in anyway, but I know something about assembling tanks and making them.
gary