We’re fairly certain that our predecessors here at the shop did not know that the late 20th century and early 21st century would herald the era of textese (i.e., LOL, ROFL), and the blocks are shelved with blocks that are the same size and typeface, carved for the other months of the year, so we’re pretty sure that MCH, like MAR, stands for March, but we’d never seen such an abbreviation before.
Historically, the shop used these calendar blocks as part of the typesetting work that would have been necessary for the billboard sized advertisements, or multi-sheets as we call them, since they call for printing on multiple sheets of paper and then pasting all of those sheets next to each other to create the final advertisement. Prior to World War II, a lot of live entertainment shows would perform multiple nights in a row (even staying in one spot for a couple of weeks at a time, just like fairs do today!), and the ads would need to go up well enough in advance to drum up business, which might mean a month or so before the arrival of the actual show in town. To save space, the abbreviations of the months were used, with the three letters for each month carved into a solid block.
A search of the world wide web reports that MCH is the abbreviation that the British National Rail uses for March, but that is as far as we could get before it was time to get back to making posters! If you have any information to share, just let us know!