Hi folks, new member. I've been corresponding with XS re their tritium products for S&W revolvers. They do not make any tritium rears, just the standard dot and big dot fronts. Although the DX fronts are not on their website, the tech assures me they exist. The Meprolight rears ARE available WITHOUT the glue-in insert, so you don't have to waste money buying a piece you won't use.
In order to get tritium on both ends for my 625JM, I'm going to purchase the Meprolight rear and the XS front, and install the rear on a spare sight body. I'm also going to find or turn a socket head screw for the front hole of the rear sight, because I use the revolver for both the revolver class at our local LE matches plus duty (night shift), off-duty and home defense. I have a Weigand rear blade and plain black Patridge on the gun now (match) and will shift both sights to tritium for carry. You can remove just the front screw and change out the rear body without losing zero - just slide the T-stud out and don't let it get turned while it's out of the gun.
For the poster who wanted paintable tritium, don't despair, they do make a phosphorescent paint (that must be what you were thinking about) - just be careful with it. You put some on your sights, protect it with a clear overcoat, and charge it with a light source. It works as well as the dots on the pre-tritium GI lensatic compass. In fact, since the tritium is dead in both my compasses, I wish they WERE the chargeable ones instead of tritium!