I still have one. We got it as a free gift with a big set of "waterless cookwear" almost 25 years ago. The whole thing is aluminum, but the bottom is thicker, to control the heat a little better. It has an insert with holes, to allow the steam to circulate. There are two vent holes in the top, that can be opened or closed, depending if you want to steam or crisp. Most of the time nowadays, though, I just wrap buns in wax paper and put them in the microwave on the lowest sitting for about 30 seconds.
Also have a built in food warmer next to my Five Star range that supposedly works well for bun warmning. Never used it for that, though, since it takes a long time to warm up.