Interesting howdah pistol.

I did see a Wilkinson howdah pistol with a side lever go through Christies in 2003. Rather poor photo of it:

Here is a link to it:

http://www.christies.com/LotFinder/lot_details.aspx?pos=1&intObjectID=4178631&sid=

The proofs on your pistol are pre-1887.

Thanks for posting.
Curl