Williams make TWO different versions of the FoolProof (FP) receiver sights for Marlins.
The older FP-94/36 fit the two factory prepped holes in the receiver side, and is a side-mount sight.
The newer FP-336 is an identical sight that has a different base, which top mounts in the two rear scope mount holes atop the receiver.
The horizontal sight bar lies behind the base, ILO on top of it, and can sit almost touching the receiver top.
The less expensive, grubscrew adjustable, Williams 5D receiver sight is made like the FP.
A 3rd Williams peep, which fits the scope mount holes, is the top-mount Williams Guide Receiver Sight (WGRS-336) - which is MUCH higher than the FP, and will need an even taller front sight blade to achieve a proper zero.
Sights that hang off the back of an M1894 do not hang off a M336 or M1895, because those models have a longer receiver than an 1894 - the scope mount holes are not as close to the hammer. ( I have both models)
Williams also sells FireSight sets, with an FP receiver sight and a proper height FireSight fiber-optic front sight (which can be bought separately in several different heights).
Here's the two different Williams FireSight set item numbers for the Marlin 336, from Williams:
Item #70861 - 5D-336 - $52.95 (Mounts using scope mount holes)
Item #70282 - Marliln FP336 - $82.95 (uses rear scope mount holes)
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