I've got a 1980 US27 that I rebuilt from a junker. In my photobucket album there are some overhaul pics sprinkled in with other pics of all kinds of stuff. You name it, and I've probably done it on my boat.
As far as a US20, I've never heard of one. More likely it's a US18 or US22.
Pictures by US27inKS - Photobucket