Might be cheaper and easier, not to mention more functional, to replace the 500 with an A5. Browning only cataloged the A500 for two years. Every example I have ever seen, and, it has been YEARS since I’ve seen one, was a really nice single shot.
I totally get wanting to fix up Dad’s old gun, but, do that knowing what you have isn’t all that good a gun.

I’ll keep my eyes open, anyway.

Good luck.

Best,
Ted