Comment by sandbach
Another detail you didn't mention: knopp or knoppe is a Low German (northern German) variant of Standard German Knopf. That's where the pf--pp alternation arises.
Another detail you didn't mention: knopp or knoppe is a Low German (northern German) variant of Standard German Knopf. That's where the pf--pp alternation arises.
Knopp is actually common in southern German dialects as well, including palatinate and hessian. Using -ke for diminutives is common across Westphalia, especially the Munster region.