Whitworth was in common use until about 1878 or so. This was followed by Siemens-Martin steel, but this dating would have varied by maker, dependent on suppliers, stock on hand, price point, etc.

According to Greener, if those are Whitworth barrels, there should be a wheatsheaf mark on them.

Here is the link to the recent thread on steel barrels.
http://www.doublegunshop.com/forums/ubbt...;gonew=1#UNREAD

With that load, this was a solidly built gun.

Pete