LENZ made the first batch of 55020 decoders and they had the LENZ logo etched on the board. These decoders were very limited in what they could do. (no BEMF for one)
Then came Massoth:
55020 Still no BEMF), no logo on the board
55021 ( now you have BEMF ) 28 speed steps OK but 128 is dependent on decoder rev in CV 7
55027 Now you can get more function keys and not all did 128 speed steps, again version is in CV 7
On board with 4 4 pin connectors in the middle of the board, 2 for motors, one for switch panel, one (usually red ) for SUSI control of sound boards.
Small blue connector is track power for jack at fornt and rear of engines for car lights or sound cars.
Load of functions and voltages is in CV 106 (read only).
Special boards were made for the track cleaner, porters and FRR engines.