You're probably close. Here's a thought, back when we and the NSSA artillery disc. boards were discussing "rifled howitzers" I looked at some old Navy ledgers of "miscellaneous guns" in the Nat. Archives. Listed under the Phila. Shipyard, I found five Blakely mountain guns, as I recall 2.9 inch. Even though this photo was taken at Washington, I'll bet the five were sent there and those are them. If so that's be kind of exciting, this would be the only known photo of a Blakely mountain gun on an original carriage (if indeed that's what those are, and not something made up at the Navy Yard.