I have a 1973 1800ES with a BW35 slush box. If you look at the underside of the differential, between the drain plug and the inspection plate, there are what looks like small cooling fins that are part of the casting.

Now, I had a parts car (probably an early 1972) that I pulled all of the M41 stuff out of, including the rear axle. I intend to convert the BW35 in my 1973 to an M41. I grabbed the axle out of the M41 car in case I don't like running the M41 with the existing 4.10. But when I look at the diff that came out of the 72 parts car, the casting doesn't have the same cooling fin things - it's just flat. When I pulled the axle out of the parts car it all looked to be a proper install, like it had never been messed with. The parts car axle is all covered with dirty hard grease so I haven't read the ratio tag yet.

So did volvo change the casting between 72 and 73? Or was the difference in casting driven by the different ratio within? Or was the parts car retrofitted with an axle from something else like a 122? Or was the parts car built so early in the production run that Volvo was using up some old axles before they went to the "finned" version?

Any insight is appreciated. Thanks.