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This is a New England Custom Gun Service, Ltd #N-100 peep sight for Ruger No.1 and M77. The sight is a receiver mount adjustable peep sight with two apertures, 0.093 inch and 0.125 inch. The sight mounts to the rear scope mount point machining with two knurled knobs driving crescent moon dadoes into the machined rear scope sight mounts. There is a depression machined in the top of the scope mount point into which another dado inserts for positive locating. Windage is adjusted using the two knobs, the right has a hex head lock screw. Elevation is adjusted with a slotted screw in the top of the sight, loosening the aperture frees the screw.
The sight is robust in construction and pretty much idiot proof. It will certainly withstand the significant recoil of this rifle and looks to be capable of the insults available during hunting. The machining is precise, it takes care to align all the mating surfaces, it simply will not go together if everything is not precisely mated. I used the Ruger back-up leaf sight to set windage and elevation during install, it looks as though with elevation bottomed out the front sight will center at around 150 yds. NECG notes in its advertisement that all Rugers shoot differently and the front sight may need replacing - they also supply those.
The Ruger flip up leaf sight might be adequate in case of scope failure but they certainly are not useful as a dedicated sight. There are quite a few calibers available in Ruger #1 that would not make a scope an iffy proposition, but two things are in operation here, recoil and the amount of drop at the speeds these rounds can be fired at.
You will find NECG here. For more on Ruger NO.1 45-70 see this.