That was the standard designation for GE dryers. I think the standard belt that is all over eBay will fit this model. Your mom's model is the 1962 "W" version of this BOL model which, I guess because of its simplicity and good construction, had a habit of lasting for years and years:
If it is the belt that goes around the drum you need Pt# WE12X36, if it has roller drive with the short V-belt take the old belt to a good hardware store and buy a similar belt.