not more rare... its from the set called "Set 81" ......like mentioned earlier.
they are blank backed
there is also the "Set 82", and "Set 83"
the so called set known as 1925-31 "Postcard Back" Exhibits is really a group of sets some with the PC back some w/o, and with roughly the same design (caption style). there are a few different types of PC backs within this "set" as well
here is good info ....see Adams post (Exhibitman)
http://www.net54baseball.com/showthread.php?t=133443