Hello Tom
Not quite an engineer Tom, only made it through pre-engineering before life happened and I had to quit. I am a machinist by trade, gun crank by preference. I took my old Hollenbeck down today, and there are two Hollenbeck 3 barrels on Gunbroker currently with excellent pictures of all components. One original and one marked 12/12/22 mag. Something fishy there that round emerged in 1959. You were right the 3 barrel is quite a bit deeper in the receiver under the barrels. I photographed my Hollenbeck 10 today. I'll try to post the pictures and see what the gang thinks of it. I am not one to work on rare original equipment unless it is pretty much headed for the scrap heap anyway. What would we do without neat old guns?