I have wondered about this a bit, I have seen that heel tooth on many many razors of mine, but only on those that also show signs of very heavy wear or reworking. I suspect 99% it was caused over time and not be design, but as Zak says, it's hard to say without hard evidence. The vast majority of depictions of period razors, as well as those in closest to original condition, there is a nice smooth line along the edge. The difference is with these razors, there is almost no transition at the tang, so any grinding work done, if not done to the whole tang as well (which I have also seen on some examples), will result in the odd shapes observed.

@rideon, I'm positive those scales are more modern based on the shape of them. They look like gutta percha but I can't say without seeing them closely.