awesome advice and references. i knew about k&k and have bought from them in the past, but they have only a used one for >$1k. i found a set locally for $600 with enough meat on them to machine to spec. for anyone reading this in future, yes steel will fit (eg. from a ponton), but aluminum is correct for SL (ponton also has an aluminum part number as i listed above, so ponton had both steel and aluminum, and our SL's only used aluminum - now you know). the reasons are a) weight and b) heat dissipation. the Big blue book says new, they would be 230mm ID and you can machine them down only to 231mm (or 6.060 and 6.090" respectively).
If steel you could go another 1mm, but steel isn't right for the SL (although it would 'fit' it is not recommended).
book-marking jgrelining. thank you!