I noticed that pg_verify_checksum takes an "-o oid" argument to only check the relation with that OID; but that's wrong, because the number is a relfilenode, not an OID (since it's compared to the on-disk file name). I would suggest changing everything to clarify that it's a pg_class.relfilenode value, otherwise it's going to be very confusing. Maybe use "-f filenode" if -f is available?
I see this mistake/misunderstanding enough that I'd quite like to change how we generate relfilenode IDs, making them totally independent of the oid space.
Unsure how practical it is, but it'd be so nice to get rid of that trap.
