Same thing happened to me. My .35 didn't have the old mounting holes on the side. Wanted to mount my Lyman peep on it, but put a Williams 5D utilizing the top two rear scope screw holes. Works like a champ, although a prefer the Lyman. The Williams works just as well though. I didn't want a rookie gunsmith to screw up mounting the Lyman.
I put the Williams on it, and after some range use.... I recommend the Williams!
I guess Marlin doesnt think many people use peeps anymore. Williams made a smart move to corner the market for peeps that is STILL around.
Four days ago I went to the range and got consistant less than and inch group at 50 yards. At 100 yards I obtained average three-four shot groups at 1 1/2" This was with the rear Williams peep bottomed out. At 50 yards the group was dead center. At 100 yards it was three inches high. No biggie. I want to put Williams fire sight on the front. Will knock out the existing front sight and "mike" the hight and width, and will go with the next highest fire sight.
Last year I had a Luepold 3-9 X 40 on it for hunting season. Felt weird to me with a scope. Decided to go back the the old fashioned but effective aperture sight! Hope this helps.
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PS: My initial sighting in was using the target aperture. Will use the larger hunting one for it's intended purpose!