I own two Hollenbeck double guns, a very early Hollenbeck (un-marked but high-grade) straight gripped 12-bore; and another 12-bore Grade A double marked Three Barrel Gun Company that shipped in 1908. Both are quality pieces; and internally there are no differences I can discern between the two guns. But first let me preface the following remarks by stating that I am NOT a gun mechanic, and thus may not be qualified to even have an opinion; but when I compare these late Hollenbeck designed mechanisms to his earlier Hollenbeck/Syracuse designed guns the obvious differences are seen in the top bolt fastener improvements the cocking device, and primarily in the cocking device. The early Hollenbeck/Syracuse cocking mechanism consisted of a cocking hook milled into each side of an extra wide barrel lug that served to push the cocking rod upwards to compress the hammer springs and cock the hammers. This method accomplished the task, but to work the barrel lug had to first push the cocking rods backwards where they would spring forward as the end of the rods cleared the top of the cocking hooks. The result was that lots of force was required to either place or remove a set of barrels from or onto the gun frame. Unfortunately the friction/rubbing of the ends of the cocking rods against the front edge of the cocking hooks at this key point of contact caused excessive wear in that area and sometimes broke the end of a cocking rod. When George Horne redesigned the SAC barrel lug in 1903 so that the milled cocking hook was replaced with a sliding cocking plate actuated by the attachment or removal of the fore iron, that problem was resolved; but this change was made near the end of the company's life in early 1905, so only about 5-6K guns were ever manufactured with this improvement.
When Frank Hollenbeck patented the top bolt for his last Hollenbeck gun, that device was apparently intended to be used on the Baltimore Arms Co gun, and it was; but his patent wasn't assigned to the Baltimore Arms Company. I suspect he didn't do so because he was already working on his new Hollenbeck gun design and was reserving his new bolting design for that gun. But the largest difference between the two guns is seen in the barrel lug. The barrel lug on Hollenbeck's last design is even wider that the lug used on his Hollenbeck/Syracuse gun and houses a spring operated rotating cam actuated by the attachment and or removal of the fore iron. With the fore iron attached, the cam is rotated so that the cocking device comes into correct relation to the end of the cocking rod, then cocks the hammers when the barrels are lowered. Removal of the fore iron releases the cocking device from contact with the cocking hooks and the gun is easily and noiselessly broken down. These guns must have been expensive to build, as their lowest priced model in the early catalog I've seen was listed at $65; and this at a time when Smith OO grades and Ithaca field grade guns could be had for less than $30, so little wonder Hollenbeck's final gun was a marketing failure in an age when double gun competition was so fierce.
At this point I don't own an example of a Royal Arms Company double, the final iteration of the Hollenbeck double gun, for examination; but based on photos I've seen, the location of the sear pin on that model is changed from those of the Hollenbeck and Three Barrel Gun versions, so obviously the internal mechanisms of those models have been modified to some unknown degree. Perhaps a V spring was replaced with a coil spring; I have no idea, but am certain someone here can enlighten the curious?
