To my knowledge, there is no double made that the lever does not get held open when the barrels are open. The means of how the lever is held open will vary based on the design of the gun.
Both the Parker and the Fox operate on a spring loaded trip pin under the locking mechanism. When opened, the pin is driven up into a notch in the bolt to hold the lever open. And then when the barrels are closed, the trip is released from the bolt allowing the lever to "snap" to the locked position.
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B. Dudley
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