The Compact Jr. we had in the mid 70's had size B78-13 tires. I remember them as being 4 ply nylon cord construction. The sidewalls were cracked when we bought the trailer and after two trips zig zagging across the country, coast to coast, semi-full timing we
sold the trailer five or six years later with the same tires.
The guy who bought it was a recently retired postmaster who could obviously afford better, but he and his wife were so excited they emptied their wallets on a Sunday afternoon to hold it until the bank opened on Monday. Such is one of the joys of life that we witnessed that day and have experienced ourselves, in a measure; that of being satisfied with less than we can afford.