Lots of questions there Ape. Basically XX7 and XU3 are both sandman codes applicable after about 10/75 (prior to that just xx7 with commercial tyres and normal GVM) The difference after about 10/75 was passenger tyres fitted as standard to sandman and lower GVM as code for xx7 and option to fit commercial tyres and therefore up GVM-code XU3.

My understanding is that if a non sandman got passenger tyres (by original owner ticking the option box) then it was designated XU4.

Back in the day the advantage of having a lower GVM was cheaper rego, so I believe this was a sales point of offering the sandman- a recreational vehicle-with passenger tryes and who cares if you could only carry 500kgs and not 750kg.