Please note the pdf below. It is a 1989 document by BMW North America noting an update to the Vehicle Identification Manual. It was a 48 page doc, but shortened it include only 1966-1976 info. You will note that nowhere is E10 mentioned.
Also attached a few pics. From 1971(?) to 1973 an E-Code sticker (my name for it) was placed on Euro '02s, usually above the VIN plate. All '02 variants had this and it noted TYP 114 on all, the exception being the Touring which showed E6. In 1974 the stickers did not show 114 but noted the car model, Cabrio, 2002tii, Turbo etc. None of these stickers showed E10.
As noted by Andrew above, we have no genuine BMW documentation showing the use of E10, and, BMW Archiv personnel have stated BMW never used that number.
I seem to remember E10 popping up in the early 2000s, usually on a German BMW '02 'site' I was looking at.
Unfortunately, this E10 myth has gone on too long now; the young '02ers have taken it on and, well, that is that I guess!
Scan_0016.pdf