That was how our A/R math calculated out too. Its the closest housing I've seen to the 13cm SS dimensionally.
Dan, your SS housing would work on it just fine.
The housing isn't a B/W casting Forrest so its cast flat and with enough material to finish machine as a 3rd gen. We turn them to HX-40 when we need them. We also turn the full and half marmon's into usable sizes quite often.