Studying the drawing there would seem to be nothing stopping the regulator rod coming out ! In theory undoing the backhead packing gland should allow the rod to be removed for inspection. The wetheader is screwed into the 10G smokebox tube plate 1/2″26tpi this in turn has the mainsteam pipe screwed into it 3/8″ 26 and using the same size thread is screwed into the regulator body. I would assume that assembly would be a case of fitting the regulator in the steam dome, screwing the mainsteam pipe in may be with a touch of sealer then fitting the regulator rod and finally the wet header. This would be fine when new and the threads new and clean . There is a small bush in the bottom of the regulator column that holds the in board end of the regulator rod, this and the packing gland may be the cause of the problem. To remove the regulator may be a problem, requiring the removal of the all the smokebox pipe work, superheaters Etc, then the wetheader and main steam pipe. Good luck Noel.

I have just re read your post, I fancy the problem may be as simple as the packing gland is to tight ? The backhead bush is screwed into the backhead 3/8″ 26 if this is loose then it will leak, tighten and seal or make a new one and redrill for 7/16″ 26. Re pack the gland may be with ptfe tape, clean the rod and refit. The rod has a small square on it that must line up with the square in the crank. A test of the regulator being free would be to make rod of 5/32″with a 1/8″ dia X 1/4″ long section on the end and then an 1/8″ square X 1/8″ long, then about 10″ long and bend at 90* to form a dummy rod and see if the regulator will move. N.

Thanks ! It is always good to know that an idea or plan has worked and all is now well. Noel.