Hey Mike,
Well whadaya know, I’ve been lugging around this part in my junk box for perhaps 20 years now and during a sort out this morning, looking for something else, I found it!
Yes you guessed, it’s a ¼ inch Female to 3/8 inch Male tripod adapter! I had a quick look at the Manfrotto 2006/7 Catalogue and it looks like they still sell these on Page 56, Part Number 088LBP.
Here is a photo of my rather old model, and a screen capture from the Manfrotto catalogue.
Cheers
Dennis