I have the engine of my '75 300D on a stand, and have a block heater to install. The heater is threaded, with a threaded bushing for a larger hole. The plugs are not the "soft" freeze plugs as I have seen on other engines, they appear to be cast iron threaded plugs, with a 19mm Allen socket inset in them. There are 4 on the passenger side. The driver side has two smaller ones. I cannot get them to budge with a breaker bar and a cheater bar!

Are these supposed to be there, and if so, how do they pop out if the block freezes? Are they threaded, or is there some trick I am supposed to know?

I wish I could offer you some advise on this subject, but I have been the same place you are now. I finally just gave up when the hex started just reaming out the the cap.

The so called freeze plugs(actualy called welsh plugs) are not there on any engine to pop out if some dim wit forgets to put antifreeze in. They are there for sand core removal in the engine block casting process, yes they do pop out when a block is frozen but that is not there purpose.and they offer very little if any protection to keep a frozen block from cracking........
William Rogers.......