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OTC Crate LS6 engine assemblys, got the cast iron OTC crate LS7 Open Chamber head installed i think mid to late 80,s . I heard that GM did this to lower the 11. to 1 Compression to around 9.6 to 1 because of piston damage because of diminishing poor fuel octane rating.
The casting number is 6272990 for the LS7 Cast Iron open chamber heads. I personaly have taken apart a few later model LS6 crate engine assemblys and they had the LS7 open chamber head.

Hope this Helps.