Good work on that Skull - very thorough.
...and the other point is that while the frames may have evolved quite a bit, the engine are externally nearly identical (I do realise that there are major internal differences) and most of the running gear, save the 34-35 mm fork change in 1977 - is similar over a 15 year production run. That is nearly unique in motorcycles (except of course for the British brands) - and it does make is easier to see why people could get lulled into thinking that nothing changed at all.
If Airtech had accepted your return they would be obliged to give you a refund or offer alternative part... sending back the package without their consent is interpreted as you trying to force their hand.