Mystery musket

[Anzac15]

Picked this up Friday at a local antique arms show. Normally I'd walk right by something like this (cut down/altered muskets are everywhere here) but it had some interesting features, so it came home with me. Features are:
Odd serial number
Gold or brass marking on barrel
Small trapdoor in buttplate
And discovered later, what appears to be '1798' on the plate. May or may not be remnants of an East India Trading Co heart next to that (this is a flint to percussion conversion)

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