Yes the Knight are EV OEM.
To call those drivers
triaxial is to give the midrange whizzer a lot of credit
FWIW, the coax horn looks to me to be skinnier than the 'standard' T-35 family diffraction horn -- but early TRX coaxes had a skinnier horn on the treble driver, e.g., the 15TRXB photo posted below the one borrowed from the OP.
15TRXBfront by
Mark Hardy, on Flickr
My
guess would be that the bottom cutout in the top cabinet was meant to be a vent and the top one was meant for a treble horn -- which might or might not be present (anymore?) given the presence of a coaxial driver.
I have no idea about the bottom cabinet, FWIW -- were it me, I'd start leafing through catalogs at
www.alliedcatalogs.com starting in the mid 50s into the early 60s. I'd also peruse similar vintages of
Audio,
Stereo Review and
High Fidelity at
www.americanradiohistory.com Their databases are searchable, but I am not sure what I'd search
for, so it would pretty much be a needle in a haystack sort of quest.