Just a thought...I have seen finials for curtain rods in glass. Also wondering if this could have been a glass fitting for a boat light. Seems like I recall that prior to the ubiquitous days of plastic, they were glass.
Something like this...modern example:
https://www.westmarine.com/buy/perko--stow-a-way-fresnel-all-round-navigation-pole-light-26-5-8--P009_277_001_016?pCode=281170&mrkgcl=481&mrkgadid=3090701155&cm_mmc=PS-_-Google-_-GSC%3ENonB%3EProduct%2520Type-_-281170&product_id=281170&adpos=1o9&creative=108421552324&device=c&matchtype=&network=g&gclid=EAIaIQobChMI-J2V--2o4wIVBIbICh1UpA_8EAQYCSABEgJqhvD_BwE