Found it!! I had my original Hackney target rifle hanging in the Sheriff's Office for maybe 15 years before I retired. Some numbnut knocked it over and broke the original "globe" type sight plumb in two. Beyond my skillset to fix, I had the parts in my desk drawer for years, then, when I cleaned out my desk the parts got misplaced - in two places. 19 years missing.
I found the cover part a few days ago, but had given up hope of finding the base. Dug out a cigar box of M/L stuff and ,,,there was the base. The post was lost long ago, but at least what I have left is fixable to restore the rifle back to original. Now to find someone to fashion the post and silver solder the two pieces back together. ...