Hi Rob,
I have seen 4313B picture of 476 & PTH90 horn,
http://www.audioheritage.org/vbullet...l=1#post344169
but never seen any measurements. May be on some other post (if any)?
May be long throat of the PT-H90 horn would not allow High-frequency dispersion of the 476 driver, but on the other side a kind of
"soft dispersion slot shape horn throat" would improve horn dispersion.
Reagrds
Ivica