Seat clamp will be a different size than the seat post... you will not find a 27.2 seat clamp. Measure where the seat post clamp would go in mm and divide by 3.14.

do not know what you mean by bolt-on, but if its a a ring holding it to the down tube than its a clamp-on if not and its only able to be relocated vertically then its a braze-on. Unless its a braze-on adapter, but thats just stupid or for someone cheap.