The original Klipschorn had a woofer baffle opening of roughly 6" X 12". The baffle opening was cut in half at some point (probably in the 50's) to improve the upper frequency response of the bass horn.
I believe the first horn section in the Klipschorn (between the baffle opening and the first bend) does flare faster than the last two sections, but I can't be sure. My understanding of the purpose of a "rubber throat" was to control the horn resistance in a way such that it results in a flatter passband repsonse, but I've never seen any experimental evidence to suggests that it actually works.
dbspl