There is a French marked piece (Correvon A Yverdon) which matches what you've described. It features a bifurcated hammer nose which allows for two nipples and two percussion caps for the same powder chamber on the single shot pistol (guess they wanted to ensure ignition). A long bayonet is mounted beneath the barrel and trigger guard with its point held by a catch in the pistol grip. There is a button on the pistol grip which is located right above the catch. When depressed, a heavy spring allows the bayonet to spring outwards.
Can you provide more details? Is the bayonet slung parallel to the barrel? Is the lock a backaction type? Where is the trigger which releases the bayonet located?
Source: Lewis Winant's, "Firearms Curiosa", pages 226-229.
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