Army Form WO.967 must have been commonly used. It is the document by which a man's commanding officer authorised his discharge from the militia so that he could enlist in Her Majesty's (or HIs Majesty's) Regular Forces.
The example I have chosen (which is Crown Copyright, The National Archives) dates to 1878 and was part of a print run of 16,000 which had been issued in February of that year.