Help - found really old egg

Those are old Japanese Kamados. They are clay and do not like high temp steak sears. If you keep them under 350 you should be fine. I would go with the medium "guts". If you wedge large stuff in it, the heat will expand the metal grid and crack it.

The really old eggs,like the one you described,are made out of clay(not ceramic).The clay won't hold up to the heat.I wouldn't go above 350 degrees with a clay one.

They are clay and are very brittle at high temps and easily broken.

About 8-10 years ago I picked up a kamado (which is what you probably have) at an estate sale. This was before I new that BGEs existed. It probably is easily 50-60 years old. They are much more fragile than the BGE. I have replaced the fire box and ring, which is one piece from kamado, once because the original and then the replacement broke apart from moving once and from heat the second time. I talked with BGE and concluded that from the measurements mine was in between a large and a medium BGE. In fact initially I tried a large fire box and it was too big. So I put the medium box and ring in that leaves a little eggstra air room between the box/ring and the shell which is probably good for the shell. I use it regularly and even occasionally when I don't watch it, it will jump up to 450-500 when I'm heating it up and it survives OK. I don't really cook over about 350 though. I even drilled a hole in it to put a thermometer in so I know better what it's doing. I now have a large BGE and just got a mini for the wife for Christmas. So I painted the kamado green to match the others and for protection. The kamado probably won't last nearly as long as the egg, but it's already lasted a long time. You can't beat the price of the kamado, I'm assuming you're not paying too much, so if you need to, you can replace the guts and get a lot of use out of it.......but I guarantee you will own an egg in your future.
Sorry for the long post, but I do have first hand experience with the kamado and my two cents would be use it and have fun. You can do much of what you see posted here on it.

I have the old Kamado I bought in Japan in 1970. Mine is a large so it's the same measurements as a large Egg. I've replaced the firebox and fire ring. It's still working great. Sunday I cooked a ham and for Christmas I cooked a standing rib roast.
A good place to check on yours is Kamado King Inc. 1-800-637-0399. They still make the clay kamados. That's where I got my parts. Also try the Kamado Forum. Richard Johnson is also one of the original suppliers of the old kamados.
Good luck.