Yes, the small letter designations are usually meant to designate variations within the model group. These small letters are not displayed outward like the "SL", "SL" "D" etc.
the Adenaur sedans had many variations within the model: 300a, 300b, 300c, 300d, all in about a eight year period! The 300d (with a small case "d") was NOT a diesel in this case!
Joe Alexander
Blacklick, Ohio