The original retainers actually used weigths to close off the exhaust. That was changed to a spring design. The first three position retainer was direct, 10# and 20#. This changed (40s or 50s?) to the four-position with direct, slow direct, 10# and 20#. The "1967 three position" retainer has direct, slow direct and 20#. I would imagine the four position ones are getting rare.

Passenger cars and cabooses generally used a single pressure retainer (15# was common) since their weight did not change much, so they did not need different positions for empty and loaded.