look like the wilcox-crittendon valves i have no my 1979 boat - seven of them. they are made of bronze and almost indestructible (just don't ground them to anything). every four years or so when i haul out
for a bottom job i rebuild
each of the valves.
on the side opposite to the handle there is a locknut and a flange nut. remove the locknut, then the flange nut. there may also be a large washer. then tap out the tapered plug
with a soft mallet. clean the bore and the plug with a rag and mineral spirits and very lightly sand them. coat the plug heavily with wheel
bearing grease, put it back in the bore, and replace the nuts. tighten them enough that the handle moves stiffly. should be good for another four years.
when i first got the boat eight years ago the po had probably not serviced them ever. they were difficult to take apart - the plugs were nearly welded to the bore. but a lot of pb blaster and mallet tapping eventually separated them....