Hi and welcome to the forum.
There is not too much mystery here but you seem to have started with an incorrect premise if (as you have originally posted it under "Sheffield Hallmarks") you think it is a Sheffield mark. It is London, but for some reason is missing the uncrowned leopard's head town mark. However the form of the date letter punch is very distinctive and, in combination with the lion passant, can only be London. M is, of course, for the assay year 1887/88.
The maker is a slight problem as there are 2 possibilities both with very similar marks; it is either Jane Brownett, in business as Harris & Brownett or John Batson of John Batson & Sons. Both were in business in 1887.