[A/S] model type by registration number so i can sell 71 airstream

["BILLY RESTIVO" <billycal@xxxxxxxxxxxx>]

i am really stumped can't figure out what model airstream i have
is there a way to tell by thee serial number?
i31t1-s2240
i think it's a 71 i know it's a land yaught,
it's either 29 ft or 31 ft
it has a rear bath
dual bunks in sleeping compartment
i have the owners manuel all documentation but it doesn't say
the reg says it's 29 ft
but the owner whow has since passed away and was a engineer has noted 
in his documentation that it's 31 ft,
i need to know so i can sell
any help would be highly apreciated
thanks
billy