What does the MS symbolism on an altar cover mean?

In a former Anglican church, now used as a museum, the cover over the wooden altar has a symbol, which I want to decipher. Within a shield, in Gothic script, are the letters M and S. The M is much larger, and similar to a McDonald's M with 2 arches, but there is a solid horizontal line making its base. Around the central vertical, which separates the two arches is an S, very like the Superman symbol, and completely contained within the larger M. The shield is shown as part of a lily plant, with the stem and some lateral leaves below, and flowers emerging to the right, left and above. This emblem is duplicated, being on either side of the central IHS Christogram.

I'd like to know what M and S represent, and, if it is possible, the relevance of lilies.