Ok. It looks like the b was dropped in approximately 1965 and I'm guessing the b c d thing was something from the 50s that was sort of like a 220 vs 300 or 280 vs 300 to designate different variants of the same body style.
This one seems to cover the transition from "b" cars to the 300se and then 280 models.
(What started all this was seeing someone mention a 300SEb and my recent research tells me there really wasn't such a thing, but I could be wrong.)
Edit: This page shows a little of where it seems the b c and d were coming from. It certainly does seem like a confusing period of model names. I am guessing that the b, Sb and SEb names were some type of a carry-over from the period of these Adenauer cars: http://en.m.wikipedia.org/wiki/Mercedes-Benz_Type_300
Edit: aw Jeese... Ok. The Ponton 4-cylinder a, b, c seemed to designate a facelift version, a being just after the initial, then b, then c. And the big D always meant Diesel. Oh man.
Well, now I see why they used a little "b". So if there was a 220SEb, then should there have been a 220SEa ? Oh. Maybe not because it seems the D (diesel) ponton jumped straight from D to Db, and all the "b" Ponton seem to be 59-61 models. Hmm... Time to go back to the 220SEb research now that I believe I found a possible origin of the a, b, c in the Ponton cars.
No real mention of the "b" other than the premier of the w111 chassis cars having the "b" in their names starting in 1959. So I'm guessing now that a lot of Benz got the "b" sort of like the "stroke 8" thing happened in 1968, like a marketing or buzz word thing going along with the 1959 cars.
Still studying up on this... So what's up with those early SEc cars. Hmm... There's a video of some lady driving one around a parking lot. Typo? Edit: oh... Maybe that's just people talking about tier SE coupes or SE cabriolet cars and not really an official name. Here's the nose relevant hit when I searched for 220SEc http://wiki.mercedes-benz-classic.co...EC-Getriebe/en