Don't see or hear much about this cooking vessel, but I DO have one. It was very popular in the '50s, '60s, '70s and the '80s. Pyrex used to make one that had glass handles.
It used to come included in cookware sets or as an option to the sets. It's still a very handy and indispensable item in the kitchen for making delicate sauces and custards for cakes, pies and ice cream. It helps keep the mixture from burning, curdling or cooking too fast.
It consists of two small pots - one slightly smaller one fitting inside the other. Food in the upper pot is gently cooked by the heat and steam that comes from the boiling water in the lower pot.
Mine came with my Visions cookware that I bought in the mid '80s. It's now more or less a bowl with side handles that fits snuggly into the top of the largest sauce pot.
I STILL use it now and then for delicate sauces, puddings, custards for ice cream, etc.. What do you use yours for if you have one?
Of coarse, you can improvise by using a SS mixing bowl over a pot of boiling water to achieve similar results.