Thanks Muzz, appreciate the help. The rusted block wasn't originally coupled with the adapter ring, i just used it to prove that it fitted a turbo pattern 308. The ring has some rust so could be a marine application though. I took another pic with the gearbox outline traced in white paint pen as best as i could. I also marked the mounting holes in paint pen as well. I'm assuming the gearbox matched the round shape of the ring on its lower LHS.

Does anyone recognise that shape?

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