Just to make things more confused...
I once had an early 612 or 620 (forget which, been awhile) that had diagonally placed rigid fiberglass like the 618 photo below.
This stuff looks like Owens Corning 703 with a coat of varnish or lacquer to make it stiffer.
I'm wondering if that "brace" is there to hold a fiberglass or mineral board damping panel and not for cabinet reinforcement...or maybe it is the mineral wool panel itself!
The early cabs had very little internal damping material. In general, I have found that less is more with Altecs....add until the boom goes away. Too much and the liveliness is gone.
Check this 620 out
http://soundup.ru/images/stories/arc...ondition-7.jpg
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