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.
Alphabetic script users' handwriting does not match print in all features. Traditional German handwriting used a line like a macron over the letter 'u' to distinguish it from 'n'. Rendering this with a u-macron in print would be the height of absurdity.
I feel similarly about the assertion that the "two lines" are something that needs to be encoded, but only an expert would know for sure.
A./
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