Yeah, the 100x60 1508 must be really rare as I tried for years back decades ago to find a pair after the local Altec distributor mentioned them, but they remained only a model number until that photo surfaced awhile back. Wound up stacking 511s as a 'make do' that fortunately turned out fairly well.

Never seen any rectangular Altec cab with diagonal damping like in your 618. Is the cab a factory one? I mean the local distributor use to get cabs made locally to cut labor, shipping costs and their resident engineer for whatever reason didn't necessarily have them built to spec.

GM