100 years (approx) sn pa20568
mod 51 means that numbers cycle between 0 and 50. For example, 51 mod 51 is 0 because the numbers have completely cycled and have returned to the starting point. 52 mod 51 would be 1, 53 mod 51 would be 2, and so forth.
its a 1950 or 51 . mine starts with a 2 and its a 1953
Contact the Remington Custom Shop
Remington's website has what you are looking for.
A Remington Mod 8 in 95% conditiion is about $500.00
Bottom of receiver in front of trigger
100-1000 USD depending on specifics
Photocopies can be found on ebay.
Age.
http://stevespages.com/ipb-remington-51.html
1958 1988 mod 552
It denotes a fancier version.