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There's a fairly low priced (at least as they go on ebay these days, and at least when converted to US$) K 28/2 listed for auction from an Australian seller. Auction ends tonight/today (<5 hours from now as I post this), and currently has no bids at AU$900.
It does mention a small mark on the rear element, but nothing I can see in the picture.
RARE Pentax 28mm f2 SMC "Hollywood" lens
Just editing to add I don't know/ have any connection to the seller, or know anything particular about this item. And I won't be bidding on this one.