I always thought as Gene has indicated. I think the application refers to flat head engines with the valves in the block as in Pontiac, Packard , Oldsmobiles, Plymouth, etc. straight sixes and eights.
I can't remember what Ford V-eights were referred as but their six cylinder engine was an L head.
The Chevrolets were referred to as overhead valves. Can't remember a letter designation for them but there may have been one.
Jonda!: The auto shop instructor could not have been that stupid. Like mine, your memory may be a bit faulty on some things..
Best,
Charlie