3/8" steam return (?)

I'm hoping someone can tell me how this works (it seems to). I was inspecting a heating system for a prospective home buyer & found the 1st floor freestanding radiators had been replaced with 1-1/4" baseboard supplied by the old radiator riser on one end with a Hoffman #500(?) vent teed on the opposite end....the bottom of the tee is reduced to 3/8" copper & extends down into a tapping on the steam MAIN. The 3/8" has a trap (loop) in it by design. I turned on the heat & from a cold start the baseboard heated . Is this some method of installing baseboard to steam that anyone has seen elsewhere?...it's a first for me but it seems it was working in the short time I tested it.