"a 135i with a M-Sport package has the ///M logo on the steering wheel (and also on the shift knob and door sills right?"
Right. It's also on the wheels.
"what about a ///M Sport license plate frame?"
Definitely. (I have one.) The "///M" is exactly as on the above items. (There is no "3" or "5" etc. unless you specifically purchase that version.)
FWIW - I removed my "135i" for cosmetic reasons - just to make the car clean, and highlight the Roundel and ///M on the trunk lid.
And then I added something -
Purchased a set of black M3 floor mats (the front ones, especially, fit the 128/135 exactly). Removed the "3." **poof** Instant matching ///M Mats for a 1!!