Those "funny loop thingies" you saw on the R-15 cab ends were radio antennas.
The BMT number code was never consistently applied to rolling stock, since the majority of their fleet - BMT standards - didn't have end signs. Unless you happened to be riding on a train of Triplexes, oddball units, multisectionals, or R-16s (or if you got lucky, an occasional train of R-1s on loan or R-10s), you wouldn't see anything except route and destination signs on the car sides.
I also remember very well when the R-32s first appeared on the D. They would glide into 34th St as opposed to the R-1/9s, which lumbered, snarled, and hissed. They streaked effortlessly along that CPW express dash, whereas the R-10s thundered and the oldtimers howled - or should I say hauled? - ass.
As for the 42nd St. shuttle, chances are what you saw were the World's Fair Lo-Vs, built in 1938. They were the last IRT-designed cars.