Steam Supply Valve Re-packing?

Hi, I have a single pipe steam system that is about 100 years old. Several of the supply valves are leaking steam--i put tissue on the stem, and the tissue is fairly damp after a cycle (boiler is using a gallon a week of makeup water, and I believe leaking supply valves are a significant part of the problem)

The valves in question don't have a packing nut--the top of the valve where the stem comes through is a molded cap that threads onto the body of the valve. It has a 1 1/2" hex head, and of course, won't budge.

I am going to try a deep impact socket to see if I can get the cap off--but I am wondering...given the design of the valve--is it possible there are no servicable parts inside--e.g., no packing?

Anyone familiar with this type of valve? Any thoughts?

I figure fixing the valves as opposed to remove/replace is preferable, as it looks to me like getting the old valves/spuds off is going to be really tough--even if I crack the valves--as the radiators are recessed in the wall, and I will need to cut the enclosure framing in order to have clearance to rotate the old valve off the threads, and a new one on the tread.

Thanks