Yes. I've heard that the 717 was indeed the KC-135 originally...
I think the customer order number was an excellent system. Mind you, got a bit confusing when aircraft were being brought by the multitude by leasing companies with their own number and then operated by carriers!
Several years ago I sat in a camoflagued 707 cockpit on a trailer outside Kermit's place at Tamiami - from the reg it was an old Pan Am machine, reg might have been N707PA, probably scrapped at KMIA - only time I've ever sat in a big jet left seat. Played with the paint-worn throttles, moved the column and imagined just how many miles the old girl had done and where she'd been in her time... sad end, just a plaything for little kids - and big kids like me.
Treadders