A sign/abbreviation for "magister"

[Richard Wordingham via Unicode]

On Fri, 02 Nov 2018 08:38:45 -0700
Doug Ewell via Unicode <unicode at unicode.org> wrote:

> Do we have any other evidence of this usage, besides a single
> handwritten postcard? 

What, beyond some of us actually employing it ourselves?  I'm sure I've
seen 'William' abbreviated in print to 'Wᵐ' with some mark below, but I
couldn't lay my hands on an example.

Richard.