The plug has a definite casted boss that is threaded, and it is a Canadian truck so the block heater theory is reasonable. I have attached a close up of it, and you’ll notice something below it that I can’t explain either. It was covered in rust crystals which I assumed was from the plug leaking but on closer inspection it has almost little holes embedded in it. I will need to clean it out better to determine what it is. I added some pictures of the casting numbers and serial numbers in case I have given this 1958 235 a mistaken identity.
