AR vs E28, which is correct??

[scar]

Ok, I have a Model A whose engine number was stamped in March of 28.
Anytime that I'm talking/posting about it and I call it an AR or if I call it an E28, I get someone telling me that there is no such a thing.
I'm wanting to know which term, if either, is correct. I'm tired of getting corrected every time I talk about the car.
So, which is correct and do you have proof to back your answer?
thanks