You have it pretty much.
This catalog page probably has as much data as you will find: http://www.lansingheritage.org/image...pro/page22.jpg
The octal socket is for a plug in transformer. Altec's Peerless transformer division made some really good stuff. You'll see DIYers even using them to build phono preamps.
Cool thing about that crossover appears to be that it would allow you to overlap the filters. The uncool thing is that the fixed steps are too large for overlapping in most cases. But, the fixed steps are far more accurate than a 69 cent pot that sweeps from 80hz to 8000hz which is what 98% of the analog units out there use.
Hopefully, Old Guy will post. I'm inclined to think he's probably had "hands on" these at some point.
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