Railing end pieces, used on the 42' Corvette foredeck.
Available in FUD (plastic). 4 required, you get extras!
Offered loose here, normally part of the Cabin & Deck Fittings set.
Hole diameter is 0.055". If using 1/16" brass rod, you will need to open the holes a bit, or file down the rod ends a bit. Drilling this brittle plastic is hazardous, even a tiny bit- it wants to shatter. A 1/16" reamer would be great, or a dental rasp... otherwise, I'd file the brass rod!
See my document on working with FUD plastic: