But the next week a wag asked "OK, smarty, how many engines does a 707 have?"
and the 777 ?
Boeing just move up the numbers, 707, 727,737,747 etc.
There is a reason for no 717 that I forget, but it was restored when Boeing took over Douglas - a D.C.- 9 variant.
I don't know why they picked on a 700 series of numbers, or why the Stratocruiser was a B-377.
BelArgUSA has more or less described it all, BOAC got 707-436's - the early Long-Haul Intercontinental model with Rolls-Royce engines, and later some 707-336's with Pratt & Witney engines, and their first 747 was a747-136 - 36 being the customer number, the Rolls-Royce engined ones became 747-236's and so on.