I have an old MTB frame that has the drive side rear dropout partially filled in as if it was part of the original forging of the dropout. And, I have seen this on a few other frames -- even different manufacturers.
Clearly it was done to limit how far back the axle can go in the dropout but the question(s) is(are):
why does the frame maker want to control this detail?
why is it only on the drive side?.
=== edit ===
This is not the bike in question but it is an example of what I mean. My apologies to the site where I found this -- it was not copyrighted.