Caveat Lector wrote:
As a second quiz -- what other information is transmitted besides the main
R+L, 38 kHz DSBSC, and 19 kHz pilot carrier ?

Not necessarily anything. But optionally:

67KHz subcarrier
92KHz subcarrier
(previous two generally used for background music, data broadcasts,
narrowcast talk, or reading services for visually-impaired)
(a 41KHz subcarrier was also popular back before virtually all stations
went stereo. 41KHz SCA is not compatible with stereo.)

57KHz R(B)DS data subcarrier

Other subcarriers are possible but these are the standards for which
off-the-shelf equipment is available.

Also possible is an IBO(A)C digital signal which occupies the guard
bands at the edges of the station's channel and the adjacent channels on
either side.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com