There was a funny thread in "letters to the editor" of AW&ST about 20 years ago about aircraft designation - the system (or lack of it) in assigning the hieroglyphics.
One writer noted that all Boeing types had a "7" in the designation, going back to the 247, 307, B-17, KC-97, B-47, yada yada. Another said "Yes, but what about the 314, B-29, B-52?" etc.
Then another writer informed us that you could tell the number of engines on a Boeing jet by the designation: the 747 had four, the 737 had 3, and the 727 had 2.
(Wow, I never knew that before...)
But still another writer then asked "OK, then HOW many engines does a 707 have?"