" The No. 35 sight, seen in illustration from 1906 catalog at right, featured a slightly-elevating slide that was easily turned to one side to allow removal of the bolt. It had a specially constructed base that allowed it to be easily attached to the bolt stop. It did not interfere with loading from clips when mounted on the rifle. The sight required a front sight of a special height."
For those with an inquisitive mind, consider the Williams Foolproof Target sights that mount onto a mounting rail, like those on a Remington 540X.
Also notice the insert that fits into the front of the aperture housing. At first, I thought it was a crude rear post (?), but if you look closely at the last picture, it is an insert that sort of resembles those used on a globe sight. Not very common.
If you have further information on the No. 35, drop me a NOTE
Source: Lyman Centennial Journal 1878-1978, page 85 ASIN: B000BY58S4