Civil War Rifle w/ strange characterisitic

My dad owns a Civil War rifle but he can't find any information about it. The catch is- the hammer is placed on the bottom near the trigger. Does anyone know about this type of rifle or what it could be worth?

Underhammer.

I recently saw a J. Henry underhammer percussion buggy rifle on sale on the Gun Broker website, it was selling for $995.00. If your father has a similar one and is in the same or near same condition, your looking at a pricey rifle.

Here's the link to the one i found:http://v4.beta.gunbroker.com/Auction/ViewItem.aspx?Item=144878894#PIC