knob

I was back in an old photographic foraging ground today - outside a frat house, where this fine knob had been abandoned, attached to a rather battered door (such is my house-awareness that I contemplated unscrewing it, wondering if it would look good anywhere around the Unsaleable).  (I am not sure whether that last word has two "e"s in it, or not).   I've done a poor job this semester in teaching my C20th British lit class British slang (curiously, it's turned out to be more literary-oriented than cultural-history oriented, unlike the one I taught last year).   But had I done so, I could have brought in the expression "with knobs on," which curiously isn't an Americanism - the equivalent would be "with bells on."   Why "knobs"?   On-line sources suggest either the tops of bedposts, which doesn't seem very likely, or "penises," which really doesn't make sense, unless one's speaking of some extraordinarily well endowed imaginary dervish.   Indeed, bells would make a whole lot more sense ...