Thanks rjscoder & zanary.
Two different, well informed Tesla owners with two different theories on HW3 & MCU1.
At least I now know the HW3 upgrade isn't currently being offered for MCU1 owners.
This is interesting - especially to me!
Hw3/ap3 is offered for FSD owners with MCU1-ap2.5 cars, with infotainment(mcu2) upgrade, so I'm not sure if you are mixing terms or not now?