riff raff:-
Contrary to what many might believe, every commercial passenger jet produced is actually a custom product.
Exactly right! I was once employed by a charter airline that flew "pre-owned" BAC 1-11's. Our fleet included 200, 300, 400, and 500 series aircraft, but within those different series were different designations, ie 207, 301, 401, 414, 509, 518, etc, each one indicating an original airline's bespoke model (no doubt the cognoscenti will guess the airline in question ;-). The variations could even extend to the performance parameters, ie the 500's had "new" or "old " wing leading edges and thus differing ODMs.
These discussions often degenerate into using cars as examples of generic "off the shelf". I'm not sure that can be true either. I once bought a UK spec VW Polo in Berlin (when such practice saved considerable sums of money, as perhaps it may well do again in a few years time). I could specify any engine and any trim that I wished, and had to avoid the temptation to stray from the very restricted range then on offer within the UK (with eventual sale/trade-in in mind). In other words, the shelf has now many more possibilities than it perhaps once did...