Do they look like this:

1953 side engine mounts

This block came out of a 1953 car with powerglide, the first 235 to have pressurized oiling. The holes here were for side engine mounts. I don't know if all blocks got the holes after that time or not. My 1957 Truck motor does not have them. I might hazard a guess that they are only on blocks for cars not trucks.