Alphabetic script users write things the way they are spelled and spell things the way they are written. The abbreviation in question as written consists of three recognizable symbols. An "M", a superscript "r", and an equal sign (= two lines). It can be printed, handwritten, or in fraktur; it will still consist of those same three recognizable symbols.
We're supposed to be preserving the past, not editing it or revising it.
