Wierd bird in Spokane........haave you ever seen this?

Hi guys.....Don in Spokaloo here........had a customer call us for the first time with an old 1900's steam system.  Asked me to come and have a look and see what I thought.  I was expecting to find either a trusty old 1 pipe, or the more common 2 pipe.  System running on a vaporstat, and all rads that have vents are working well, but there are 4 or 5 rads with 2 pipes going to them, and a wierd looking "trap looking" contraption that is piped in where a vent would be if it were one pipe.  The thread it attaches to the rad with is 1/4" and then the little trap looking thing, and then 3/8" pipe going to some kind of a return pipe going back to the boiler.  THOSE TYPES of rads are not heating up.  My first thought was "this was originally either all one pipe, or all 2 pipe"  but I've never seen this type of system before, and it was not in my customers trusty "Lost Aart of Steam Heating" book (imagine that 8-).  Here is a pic of the "little trap" looking things.  Whaddaya think?  My tech that was there yesterday is wondering if those little things are actually some kind of a "piped air vent" or something, instead of a trap.  I dunno.