Does your 36 have diamond stocks and a standard cylinder latch? Diamond stocks were phased out about 1968, I believe and the standard latch replaced the flat latch in 1966. I have a 3in 36 round butt in the 508xxx range that is just about NIB. Mine has a standard latch and diamond stocks numbered to the gun. From information I have found on this forum, I believe mine was probably shipped about 1967. I haven't bothered with a factory letter.