Join Date: Jul 2010
Location: Outside Philadelphia, PA
There are some very good reasons as to why these tanks were ditched by most people. The all glass tanks look a lot better, and are much less prone to leaking. That being said there is now sort of a revival or stainless steel framed tanks due to the retro look.
As to the tank itself, that tar like cement that was used tends to dry out after many years. Your tank is most likely leaking. As long as the slate is not cracked or damaged it should be ok. I would drain the tank, let it really dry out for a few days, and run a bead of silicon around all the joints. Be sure to get up under that top rim too. That should seal the tank, and you'll be able to use it.
As for trying to use it as a SW tank. Do not even consider this. Having been in the FW and SW hobby long enough to have actually used such tanks in SW, I can tell you they have some problems. The stainless steel used in the tanks is not that high a quality, and it is spot welded in the corners. The frame will easily rust. Way back when, when using such tanks in SW, many people painted the frame with epoxy paint, silicon, or liquid aquarium sealer. The other problem, is the tar like cement used can be toxic to SW livestock. However, if you seal the tank as I recommended, you shouldn't have that problem. If you find a stainless steel light for it. That will also rust if used around SW, for the same reasons. As a note, you have no idea of my feelings of joy when I was able to get an all glass tank for SW, and ditch the stainless steel framed tank.
If you want to go with SW, spend a few bucks and get a new tank.
To answer your specific questions about this style of tanks history, This style of slate tanks was used in the hobby from the 1950s until the end of stainless steel framed tanks. However there were some manufacturers that did use a thick piece of glass on the bottom, rather than the slate. I don't know of anyone making tempered glass bottoms in these kind of tanks, but it might have been done. It's hard to tell how old your tank is because the style was used for so long, but I'd guess it was from the lat 1960's or 1970's.
When the all glass tanks like we see today first came out, they didn't use tempered glass. This made a tank like a 55 very heavy, because the glass needed to be thicker. The tempered glass came a little later. The original AllGlass brand tanks actually used a wood bottom frame. This was about 1967 or so. They could rot over years, but did have the advantage where you could refinish it to any color you wanted. A few years after that, the switch was made to plastic frames, and we have the tanks we see today.