In 12/2021, this seems to be the nomenclature used in B+W's Übersicht Produktprogramm nach Filtertyp | Overview Product Range by Filter Type
E beidseitig einschicht vergütet | Single Coating on both sides
MRC beidseitig Multi Resistant Coating | Multi Resistant Coating on both sides
MRC nano beidseitig Multi Resistant Coating nano | Multi Resistant Coating on both sides nano
I'm not seeing any reference to uncoated filters, or different thread pitches. I don't doubt that these, and different uses of the letter E, existed in the past. Since B+W also isn't very good at dating their catalogues and handbooks, it is hard to tell how far back the current nomenclatures and product lines go. If I dig through my files I must have a variety of each that I downloaded over the years. EDIT: I looked through my files and found a 2010 B+W document with this as the E nomenclature, so it goes back at least a dozen years.
The lens caps (page 54 of hte HANDBOOK)... perplexing... wouldn't be surprised if written by a marketing person is just as confused about the B+W nomenclature as are we!
Last year I went through the same puzzlement. All I wanted was a skylight-type filter and ran into all sorts of confusion. What I bought was marked 67E (seems to be single coated and a soft coating too) in a F-pro mount (deducing from the mount pictures/descriptions). I made sure I bought it at a good enough price that even though I wasn't completely sure of what I bought, I wouldn't feel "burned" if it didn't work out.