I had two HW 55's at Findlay, one for sale and one display only.
This is the one happy to be at Chuck's house - an "S," with dark-stained beech stock, from 1965.
The display one was a VERY early HW 55 from 1953 or so, with a Bayern-style walnut target stock. I've called it an M in my caption, but actually there is no variant letter stamped on it. If you look close you can see many differences from later ARH/Beeman-era HW 55's...shorter receiver tube, cast aluminum buttplate, very different trigger and guard, longer barrel, different sights, side screws in the fore end, and more.
While HW more or less standardized details of the common variants (S, M, T) by the mid-1960's, they were notorious for ever-shifting designations over the years. At some point the S and M became the SM and MM, but to this day I have no idea what that extra M was for (it was NOT for the M's accessory rail). And as far as I know, there was never a TM!
The other 55's I saw at the show were an S with a nice action in a banged-up stock, and two T's; one with some weird bubba mods, the other in lovely original condition.