ok so todays seiko is my oldest it's a 66-9990 well they was from 1962 to 1967 I believe mine's hard to date as the system I use like's four digits at the front to work off so I have seen 62 with very low ref codes of six digits and seen 67 with 7 digit codes so mines a six and numbers in the middle so would say my gut say this is a 1965 one and it just feel right so saying this is a seiko 66-9990 from 1965 the clasp I'm not sure on looks like a replacement and fairly modern but as these probably came on leather no biggie any way