Impossible to give you a definite answer without a picture or two, or a more detailed description. The serial number you give doesn't really make any sense.
Varmint models have a 24" "C" weight barrel. THe early ones only had holes for the older target type scope bases spaced ca. 7 1/4' apart. After the mid 1970's, Ruger added a third set of scope base holes so that modern scopes could be used at the normal Ruger scope base separation distance.
The 1B has a 26" medium "B" weight barrel with a quarter rib for the scope rings.