John, I've found it seems to be more an "age" thing than anything else. So the earlier the machine, the more likely it will be unique to that Mfg - or even model.
As more machines started being outsourced, they got more and more uniform.
One interesting development was when Sears decided they wanted to sell a machine that "looked" like a Singer and used the same size shuttles/bobbins and needles. That is where the first uniformity started.