Answer: because that’s the replacement for the one I dropped overboard today!

Fortunately, I had a new/spare aboard, that was close at hand (NOT buried deep in storage.)

I knew this was going to happen *someday* … $20.

The waste cap is the one most likely to drop overboard cuz the retainer “chain” isn’t used on Waste caps … no chain enables a good seal with the ‘honey pot’s’ business end.