Gas tank cap???

[all4gss]

Hello,

i traded for an early model short bed. I'm not sure what year it is, and the guy said he got it from someone else and he isnt' sure either, anyway, the fill neck has three small holes for screws i imagine, and the bed side has the hole mounted flush, it's not inset and doesn't have a fill door. What cap do i need, looking through the catalogs i see there are different ones for 73-74 and 75-78? or something.. how do you tell?

thanks,

keith