Looking at JohnL57's lovely 55 MM reminds me of how much I like ALL HW 55's! Over the years they had 4 basic stock designs, and IMHO they put a lot of thought into each one.
The "SM" was the basic beech stock. Same basic shape as the walnut "M," but just a touch more rounded and narrow, with no sharp cheekpiece edges, etc. The economical choice that you can use to win the match down at the pub and shoot dinner on the way home, lol. This one is an ARH import with special Williams sight.
The "M" is the lovely all-rounder. Nice walnut with the classic Bayern-style cheekpiece and other German details. If I could only keep one...this might be it (it's actually the first 55 I bought). The accessory rail is a nice touch and lets you play with some cool accessories.
The "CM" Custom Match came along fairly late in the 70's, with styling obviously influenced by the top dog recoilless rifles of the day. It's much lighter and more comfortable in the hand than you might think, and the adjustable buttplate is a nice ergonomic touch. Also has the accessory rail.
And finally the "T" which has already been covered. This is a classic mid-production one with nice pale European walnut and large-pattern checkering. Just a very elegantly-proportioned rifle.