Woohoo! Look what I found - a full page ad from the Montgomery Ward catalogue from 1934. You can see here what typical undergarments for the contemporary woman looked like. Brassieres (the bra), girdles (shaping below the waist) and corselets (bra and girdle in one) were fairly new developments at that time.
You can see that the "brassieres" on this page had darts for shaping - only one used horizontal seaming. They all used hooks and eyes - no zippers. Even though zippers had been invented, they were't popular in girdles and corselets as the teeth were still made of metal.
The fabrics are mostly described as "sturdy cotton faille" and all are boned. Two of them are made of Rayon (a very new fabric at this time). I am actually surprised that these were offered in colours - tea rose and pink. Don't think I am complaining - I love pink!
Best of all - can you believe the prices?