Could have been on anything depending on their use. I would think it more likely they were permanently attached to a bench though. On a cart, you'd have to keep it from moving when you pushed on it which is possible but I wouldn't like itMore like was it on a stand/cart etc.
I bought some about 10 years ago from someone online.The plastic wheels eventually fail. Replacements were hard to find ten years ago, but someone may be offering them now.
I got these a few years ago. Not cheap, but gave my machine a new lease on life.