The "M" means MOUNTED. "M" batteries are a direct replacement allowing the removal of the factory battery box. I've used them for years on other aircraft models. "M" batteries have a mounting system designed into them with a hole on each end, so the long bolts that originally secured the battery box are reused to secure the new battery, sans $1300 fiberglass box. Clearly the way to go if your fiberglass box is kaput.
From Concorde's description: "The RG24-15M features a manifold to facilitate direct replacement of the originally designed manifold vented battery"
A picture:
https://www.pilotshop.com/catalog/gr.../11-03877a.png