Proper cow bones to prepare holodets
Not sure about Holodet. But a good source of making gelatin from meat is the knee bone with all the cartilage in it. I use it to make foot jelly (aka gala, galarita, ptcha). This is not the same as bones with marrow. AFAIK marrow is not a big source of cartilage. You do have to simmer at low heat for a long time to get the cartilage to dissolve. Don't try a rolling boil, it wont work.