Welcome to the forum.

MichiganScott gave you pretty good advice vis-a-vis a more traditional investment structure; you might reconsider your position if you're interested in a likely better return. Either way, a simple "No, thanks" will do.

I don't know of any reputable financial advisors or publications that are suggesting firearms as suggested investments; "valid" perhaps, but not suggested, except maybe by preppers.

I don't think the 640 in question is a sound investment simply because it's a total gamble trying to guess which if any of the current crop of guns will have collector's value in years to come. Your logic holds in that it'll be a less common gun, so no matter what it is likely to fetch a little more than its pedestrian counterparts 20 years from now, but trying to determine what will be collector's items in 20 years? May as well look into a crystal ball.

There's a perverse but valid argument that if the lock slowly fades out, as it appears to be in at least some of the J-frames, the guns with the lock will be the collectibles: curios from a brief, abandoned experiment in S&W history (one can hope).

That is to say, collectibility can take strange, counterintuitive turns.

I think you'd be much better off looking to ones that have already established themselves as collectible; that's the best predictor of future collectibility. At minimum, this means pre-lock and pre-MIM; ideally back to the pinned barrel era, if possible (which it should be). If you can't find an NRA "perfect" with box, papers and tools out there for $800, you aren't looking hard enough.

Hit the gun shows; look at the classifieds here; peruse the online auction and vendor sites. They are definitely out there.

Since the gun will in effect just be sitting around collecting value, it almost doesn't matter what model it is; the condition and completeness of package will be what's most important.