Are either correct for the truck? I think the fancy one is like the one i had in a 41 car and the "more square" is truck or truck only?
The defroster nob on my heater, from what i found is supposed to be a 39 40 because its just a plain nob were the replacement one available should be larger with DEF on it
The small trumpet shaped defroster knob in photo above would be correct for your 1941 Ford truck heater also...... I doubt if you will ever find any new or reproduced ones. Your only chance would be to get one off of an original heater.