What is it's purpose on the 825?
It ties the woofer baffle/motor board to the top panel of the cabinet.

Later the 828 cabinet would use a dado and triangular glue blocks on the interior side of the motor board. The differences in motorboard construction are pretty well illustrated in the various plans in the Altec Library.

The 816 drawing in the "70's enclosure plans" doesn't show blocking of any kind. When we built them we used 1 inch square hardwood blocking.