My later crankcases have the step, like yours, but the earlier ones may be different - I don't have one to hand to look at. It maybe that they "bashed" the chassis on the production line if it was needed.
But, there is also the difference between the Austin cast crankcases and the Birmal produced ones, maybe one touched and the other didn't?
I think one type has "Austin" cast on the side of the case and the other has it cast into the web but I can't remember off-hand which was which....
The cases I've just looked at, Birmal on the web, Austin on the side. I'm not even sure if this was consistent though...
But, there is also the difference between the Austin cast crankcases and the Birmal produced ones, maybe one touched and the other didn't?
I think one type has "Austin" cast on the side of the case and the other has it cast into the web but I can't remember off-hand which was which....
The cases I've just looked at, Birmal on the web, Austin on the side. I'm not even sure if this was consistent though...