I suspect Alex might be right about the enamel. I've gone through the ads and see a fair number of ads showing this case shape in gold, as well as gold plate but none enameled. Here's an unnamed ad from 1927 that is a match to identified Princines in the Db:
I'm going out on a limb here and saying it's possible this is an unadvertised Princine in solid gold grade with embedded debris in the high point worn engraving and obscurring the lowest details? It appears to be a match to this watch, with the exception of gold content.
https://www.mybulova.com/watches/1929-princine-8664
I would suggest having the case untrasonically cleaned and see what happens. Enamel will be unaffected, but ancient "cookie dough" will dissolve and go away.
Maybe I'm all wet, but this is all I can come up with.