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On ebay, a Pentax 645 A* 600mm 5.6. A little high price for the condition but these are rare. Last year I only saw 5 sell. This is the 3rd this year though (one of which I own!). I just keep hoping to find the drop in polarizer for it. I only saw one of those last year.
Pentax-A* 645 600mm F/5.6 ED (IF) Star Lens | eBay