Inside of this cabinet, there is some tannish-yellow insulation, about 1.5" thick. Is this original for this era, or did someone perhaps add it afterwards to help deaden the enclosure? Any thoughts are appreciated on this one.
Yes. Insulation in a sealed cabinet like that helps to prevent standing sound waves bouncing back and forth inside the cabinet and also, I have heard, makes the speaker respond as if it were in an even larger cabinet. I remember opening up my first Marshall cabinet and being surprised that Marshall didn't use any insulation inside. These days not every guitar cab has insulation inside because some guitar player/designers feel no insulation results in a livlier tone. Lew