Having discovered the photo in the first place and buying it on Ebay, I can tell you that there are actually quite a few a differences in this engine compared to the production version.
In the cross section drawing that appeared in one of the Hugh Conway books it was shown with a spark plug on either side of the engine and this has often been argued that it was because the prototype had been built around a redundant type 34 block. This photo proves that it was not the case and externally the block itself is the same as the production unit.
There are however a number of differences in the alluminium components, the most important one is that the upper camshaft gear box at the front of the engine is of the same width as the cam boxes themselves. On the production versions, the cam box was the same width as the engine block and thus the narrower cam boxes stepped in slightly.
another obvious difference is that the crankcase is plain, without the Bugatti logo access plates on the side.
Other differences include the fuel pumps that are located in a different place, towards the firewall and the plated tube that covers the HT leads from distributor to spark plug is in two pieces whereas the production cars have a more elegant one piece component.
The carburettor is identical to all of the cars apart from 41100, which has a strange spring arrangment mounted on top of it.
In short, this photo tells me that although there are detail differences between this prototype engine and the subsequent production cars they all seem to be cosmetic ones.
I recently posted a reply proving that the wheelbase of the prototype was the same as the production car and it was an optical illusion that made it seem longer. The prototype initially had different wheels to the later cars, this was mainly because the diameter of the brake drums needed to be increased for obvious reasons.
In short, I can understand Ettore wishing to improve braking performance but would he have reduced the engine capacity by 15 % because he wanted to improve fuel consumption or slightly decrease engine performance? I think the engine was always 12763cc and never 14726cc.
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