John,
I suspect the rarity is that there have only been so many found. I'd guess based on just what I've seen there are less than a dozen of them; I don't think I've ever even seen more than 4 or 5 with this guy having 3. I'm sure there are more than that but not many. Perhaps Mike and some of the other collectors might chime in indicating they've seen more.
As Pete theorized in his book that given the difference in shape (rounded nose) and the use of the lathe as opposed to other Archers, this pattern was very likely the product of some small deep south manufacturing site that may have only supplied troops in the south Georgia, South Alabama, Carolina and Florida where most were found (near Jacksonville).