The 510.xxxs have been discussed in the forum quite a lot. If you click on Search (in the menu bar above Message Index on the home page), change the date range to "posted between 10 March 2003 and today" instead of the default 2 weeks and search for "510 501", it should return a good page-full of posts.

They can also be found in the catalogues page of OnTheDash during the 80s:

Not something I watch for on eBay and a completed listings search didn't turn anything up, but for an idea of value, you could search in our sister ChronoTrader forum - a search there turned up a number of 510.501s over time, as well as some of the other case variants.

Mark