Howdy
Because Garner's hand is covering most of the grip, it is difficult to say exactly which model Merwin Hulbert he is holding.
What I can see is there is a top strap and there is no barrel rib. This means that it is either a Frontier Third Model or a Pocket Army Third Model. The first and second models of these revolvers had no top strap, the third models had a top strap, and the fourth models had a top strap and barrel rib.
I am lucky enough to own this pair of Frontier 3rd Models. The seven inch barrel was standard, although the MH was available with different barrel lengths. These models were chambered for a proprietary cartridge called the 44 Merwin Hulbert. They were also chambered for 44 Russian and 44-40.
This pair started out with one chambered for 44 Russian, the other chambered for 44-40. They have newly manufactured cylinders now, both chambered for 44 Russian.
It could also be a Pocket Army, Third Model. This is my Pocket Army, Second Model. Notice the lack of a top strap, the scoop flutes on the cylinder, barrel wedge, and the lack of a barrel rib. Those features define this MH as a 2nd Model. Add a top strap, loose the scoop flutes and barrel wedge and it becomes a 3rd Model. According to Art Phelps in his book The Story of Merwin Hulbert & Co. Firearms, the third models of both of these revolvers, with the top strap, and lack of the scoop flutes was first produced in 1883, and the fourth models with the barrel rib was first produced in 1887.
Although the grip shapes of these two revolvers are radically different, the portion showing above Garner's hand could be from either model.
The other thing I will say is that the position of Garner's trigger finger indicates that the gun may have been a double action. You will notice that all of my Merwins are single actions, but these models were available in double action models too.
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Yes, Jesse James' mother was well known to buy guns and pass them off as having belonged to her late son, so without a proven provenance it is difficult to state categorically what guns were actually his.
The choice of long barrels are interesting. Maybe intended for carry in pommel holsters.
7 1/2" was the only barrel length the early Colt Single Action Army revolvers were built with. Nothing to do with pommel holsters, which would define them as 'horse pistols'. The SAA was specifically designed to be a 'belt pistol' which means it was carried in a belt holster. I was not until about 1890 that 5 1/2" barrels started showing up on Colts, mostly from 7 1/2" Cavalry models that had been refurbished either by Colt or the Springfield Armory. Most of these revolvers had their barrels cut down to 5 1/2" and were issued to artillery units, hence the unofficial designation of Artillery Model.
