Dual relief case has bigger and better oilways, and 2 pressure relief valves. When the engine is in the car look up at the sump and you'll see either 1 or 2 large screws - the head is about 12mm diameter and you can use the screwdriver for opening paint tins on it
You can make a high performance motor out of a single relief case but a dual relief is a better base. My 1641 is built from a single relief case and is drilled & tapped for full flow with a filter and cooler. Runs fine
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