As Clem says, the whole E plate thing is a red herring. Their appearance on bays is down to the individual DVLA office not being being bothered about exact dates during the year. No early bay should have an E plate, they stopped issuing them on 31st July, bays were not officially available until 1st August which is exactly the date that very first F plates were issued.
Joanne Doe (38, married, 2 kids, wondering fish fingers or sausages for tea) and working as a clerk at the DVLA is not going to know and doesn't give a toss about production dates, that's for sad people like us. You put 1967 on the form, she is not interested in exact dates, so types in 1st January. The computer then says 'E' reg.....
The Speedmaster bus rolled off the line on 25th August 1967 so is very early. I remember this because Tim Glover (Roadhouse Motor Co., who imported it) checked it out and it also happens to be my birthday! (not the year, I'm slightly more vintage :lol: )