Review on Stant 12451 Cooling Pressure Radiator by James Mayberry

Part description is inadequate. It is described as an adapter but does not say it will adapt. I bought this hoping it was the right item would be to allow a stant pressure tester to seal a deep mouthed radiator filler neck from an old unpressurized system it will NOT do it it's too big as it was so cheap I didn't return it but I'd like to know what it's actually supposed to do.