Not sure there is a difference. There may be a difference that it is riveted to the frame but the mold would be the same with some small fit issues cut or added that I think once you're there will be obvious. There was a Metalcraft frame that some tubs were bolted on to.
"My parents used to take me to Lewis' department store in Edinburgh. They were skinflints, they used to take me to the pet department and tell me it was the zoo."