Interchangeable parts
I think you’ll find that a lot of parts on these different engines are shared/interchangeable. It’s kind of like a small block Chevy, where you can put a 283 and a 350 side by side and there’s no way to tell the difference between the two....they are physically the same size. The only way to determine the difference would be to measure the bore and stroke, but the physical size of the parts are the same....heads, intake manifold, oil pan, etc. Heck, there’s even some parts that are interchangeable between a straight 6 cylinder and a big block Chevy. I’m sure Lycoming, just like Chevy, is going to make as many parts as possible interchangeable....it just makes manufacturing sense. I’m no A&P, but I’d be willing to bet that you could find lots of parts off of a 235 that would bolt right up to a 540, and I’d bet that the accessory case is one of those parts.
Mark