Daniel
I've studied and collected Syracuse and Hollenbeck guns for years and can tell you it'd be a fool's errand to attempt to modify your early Hollenbeck marked frame to a 3-barrel; the Syracuse frame would lack the depth to accommodate the rifle barrel in spite of the fact that the frames appear virtually identical outwardly. And although the function of both guns have much in common, all the internal parts are different. If you are serious about a Hollenbeck 3 barrel project gun I suggest you check out Cherry's Fine Guns in Greensboro, NC. On their web site they have several Three Barrel Gun Co/Hollenbeck drillings; all projects to various degrees and priced in the $400-$1,000 range (those marked Royal Gun Co will differ to some degree externally and internally). Two things to remember with these great old guns, don't ever buy one with loose ribs (may be very difficult to regulate properly); and there is no safety for the rifle barrel (all Cherry's ads used to state that the rifle safety doesn't work) as Hollenbeck's drilling design never included a safety for the rifle barrel. Cherry's also has 2-3 Hollenbeck side x sides, and all guns listed are Damascus. These guns were carefully made, beautiful, and expensive when new; so the company didn't survive very long. Too bad a great number of the surviving examples have been neglected and abused.