Same dimensions as my two 75s that have center bracing. Do you think that someone might have removed or broke the brace? The braces are to keep the glass from bowing. I guess that some 75s might not need the braces if the glass is extra thick.
Go to a glass shop and have a peice cut to fit that is thick enough not to bow. You will have to allow for the filter, heater, ect. You will need a notch in front to feed thru or get it cut in strips and hinge the front piece. Have fun