I'll see if I can post some photos. Thanks to all for the input. Snake gun, garden gun, specimen gun, all adds up.
It's quite fine work; select wood, tight checkering, precise fit, and a respectable amount of engraving other than the side of the lock which simply says, "Marsupial Express". I'll see if I can locate the Birmingham book. I think the side lever would date it to turn of the century or slightly before but not sure about that? Still hoping Dickson and MacNaughton will respond.