I only have older Model 39a's, and haven't even looked at the new Marlin's for the last 10 years - so, does the upper left receiver wall have two small plugged holes, about 1/2" (.400") apart ?
If so, the just remove the two plug screws, clean the threads involved, and install the peep.
If no factory prep, then it'll need to be D/T - presuming the peep you bought is a side mount.
I also have no experience with the newer Williams top-mount peeps for Marlins - which normally use the two rear scope mount holes atop a receiver.
BTW - Didja remember to by a slot filler blank, for the rear sight dovetail ?
The peep'll work much better with the rear barrel sight removed to provide a clean sight picture.
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