Well, we talked a bit about this via PM, and you have already hit upon a big point -- these simply are not traded often, especially the "even more rare than rare" 6-inch long slide variant. That makes this difficult to accurately price.

Searching Gunbroker is also of very limited help, what formerly seemed to be a good database is now no longer for auctions seem to evaporate within two months or less.

The closest thing that you can compare this to is 6-inch long slide 952-2, and in the same condition that you describe, the similar 952-2 6-inch is a roughly $4,000 handgun at this time. Dead mint, NIB with outer shipping carton and end label has actually sold for over $6,500 but that was just over two years ago.

Contacting the folks I suggested would most likely bring the highest sales price, but they take a fee.

If you sell it yourself on the classifieds here on this forum, I believe that $4,000 is a fine price to ask.

Please do understand that there simply does not exist the "RIGHT" answer, it's just not possible to have an established market when these almost never appear available for sale.