nevermind Nick, he gets a little cranky now and then and thinks hes a know it all..
The Celica Supra was the original supra regardless of the Celica heritage. Does it bear much in resemblance to the mk3, not really, but neither does the mk3 to the mk4. You also made a great point in that the Supra was the "Supra" because of how much more it had over the basic Celica. If you look at the mk1 supra and compare it to the Celica GT of its era for example you have huge differences at the subframe level and up.
Because the first two gens of the Supra came from the Celica line doesnt make them any less of a Supra.
I always thought of the Toyota 2000GT starting this whole Supra deal, it even had the first M Series motor sitting in its engine bay (Funny ironic story about that too Datsun (Nissan) was originally supposed to use the newly designed M Series motor from Yamaha for their 240Z (Fairlady Z) but due to design time Datsun took to long and Toyota scooped up the engine for its newly designed 2000GT so nissan ended up using the L series engine as we all know and see today). In order to back my opinion if you look at the last Supra Generation the MKIV you can see the resemblence to the 2000GT
So in my mind the 2000GT was the first real Supra, even in Japan the Supras were usually advertised as the 3000GT (Depending on engine size of course)
Ive had the honor of seeing a real 2000GT only once in my entire life and its one of the most beautiful cars ive ever seen