Removing the rear brake drums on a 544 takes some determination. This might help.
I drilled a small hole in the drum hub just above the key and as close to the drum as possible. For a week every day I filled the hole with penetrating oil. It seeps in, follows the key and I think it helps.
I then bought a big bad puller with a cross bar that you hit with a large hammer. The first one came off with the puller alone, but the other side had to be heated with a torch to get it to pop.
put the nut on flush with end of the axle to contain the drum when it pops and to kind of chase the axle threads when you remove it.
don't give up if it won't come off the first try. Let it cool, oil it again and come back the next day with a bigger hammer and hotter torch.