My Ashland gun is SN ALU1600. My research indicates that there were only 660 of that run built. Maybe there were some more in a "use up the parts" run by S&W (it has been known to happen). The frequency of their appearance for sale is probably due to the fact that they are at first glance unique, more so than many other Smiths, and therefore many folks think they are worth big bucks. I have a 60-4 and it is by far the better packing gun IMHO.
BTW, I got mine from a fellow Forum member about 6 months ago.
IIRC the first Target Chiefs (M36) were made for HH Harris Company, a well known Chicago police supply gun shop. I remember seeing one there about 1960, he had had them for a long time, they just were not popular with the cops of the time, they were square butt, which turned me off as I (still) prefer round butt J frames.