1965 Barracuda rear defogger

All is good, but does any one know what that speaker looking plate on the passenger side under the rear corner of the passenger side back glass was used for?
It looks like a speaker cut out (6"x8" speaker) but it was never cut out on all the one's i've seen. I've got two 66' Barracudas; a /6 fully loaded car being restored now and a 66' Barracuda formula s and neither have the spot cut or filled. the fuel fill pipe is in the way on the drivers side so no "stereophonic sound" for me.