M stands for motorized, and X refers to the frame. The T refers to the head mounting. The earliest 45's had the frame adjustment bars facing forward. This got in the way of moving the easel about, and they were shifted backward. On these models, a swivel bar also allowed the enlarger to swing 90 degrees horizontal for really large projections. Yes, the models up to the T have the tapered bellows and a filter drawer. They were originally introduced in the early 1950s. You will also note that the older models are a turquoise color. All went to black in later production models, IIRC in the mid 1970s. There was a change also on how the lens and negative stage was held in alignment to the track bar. The older ones used brass shims along the back, whilst the newer models use a plastic/teflon shim.
Although the tapered models are still great enlargers (I have both), the later model with the square bellows is a better long term investment, mainly because of potential bellows problems as the enlargers age.