Can we make isoglosses?

[Anton]

That said, the use of x may point in the expletive direction

If x is interchangeable with cks then I don't see how exactly that would point to the expletive on its own. To me it looks much like the p/b interchangeability. It would be used as genitive for bock, no matter whether expletive (for God's) or literal (for goat's)

Regarding dating, just noted that the above screenshot from Idiotikon made by Aga shows various dates around 1400 for the "expletive" use of "box".