Homer, reference List of changes...31st may 1926.... Cases to be stamped '.22 ' on left side, to enable ready identification of the magazine case.
If my memory serves me correctly these were to be done with 1/4 in. stamps? someone else may have that info. at hand, mine is in use elsewhere at the moment.
A reference to this can be found in Skennerton's TLE page 287 para.4