It could be a thread mount version. I understand that lenses from about the 16xxxxxxx serial number range were either sold as thread mount or had an M bayonet mount attached with a grub screw. Later lenses were only available in M mount.
I wanted one of these transitional lenses to use on my Barnacks, but the prices were so outrageous that I finally settled for an M mount one for about $250. The curious part is that the 135mm Elmar lenses were also supplied in both thread and M mount and the thread mount ones go for very short money. It's a wonderful lens, BTW.