If you mount a receiver-top scope on the roof of your Marlin lever-gun and need to still cock your hammer to fire, you are going to need an extension as the eyepiece is going to come back too far to squeeze any but the narrowest of thumbs into. Marlin has for a long time shipped the proper extension inside a small plastic bag stapled to the interior of the box. If your rifle is missing the box or was bought new, you may be missing it and have to find one. This short article will help you find which one you need.
Old Style Marlin extensions
Marlin realized that their customers would want to mount scopes atop their receiver in the late 1940s when optics first became affordable and wide spread among the average sportsman. It was at about that time that they began shipping a flat hammer spur extension to allow these 1957-1982 made rifles to fire once optics were fitted. The old-style flat hammer spurs will be marked 'JM' on the top as proof that they are factory originals. However, these extensions were replaced in 1983 when Marlin introduced the cross bolt safety to their rifles and the hammers were redesigned.
There is a way to get these old flat extensions to work on newer post-1984 Marlins. From one board: "The flat hammer spurs were put on pre-safety Marlins. When the safety was added, the trigger geometry was changed and the round spurs were introduced for these new hammers. The trouble with the round ones is they have a tendency to shoot loose. The fix for this is to replace that tiny retaining screw with a 4-40 screw 3/8" - 1/2" long. "
Uncle Mike's makes a round pre-safety spur that is cut for these older Marlins and will give you the look of the new spur on the old rifle.
New Style Marlin extensions
In 1984 with the new 336CS equipped with the cross bolt safety, the hammer spur extensions were round and smooth. Don't know what year your Marlin was made? Starting in 1973, the year of manufacture can be determined by subtracting the first two digits of the serial number from 100: Example: SN 2512345 would have been made in 1975 [100 - 25 = 75.] So keep that in mind when looking for that extension.
The problem is that not every single marlin lever action hammer has a hammer extension out there made for it. Older guns, usually pre-1956s made 336s, were made with a hammer that was too thick for either the old flat style or new round style to work. For these you have to look to aftermarket so-called 'universal spurs.' One of the most commonly used ones is the Carlson's Universal Hammer Spur Extension, sold for about $12 has knurled ends for a positive grip and looks completely unlike any Marlin factory extension. It's made with hard-anodized aluminum and looks pretty hard to break. It installs easily by simply sliding the extension over the hammer and tightening the setscrew with the included hex wrench. Unfortunately, on the newer Marlin model 336 at least it has been reported to be sloppy and has some wiggle to it. However, on some of those early Model 336s...maybe you have a winner.
The Williams Gunsight company has for decades made rear peep sights for Marlin rifles that have necessitated the use of a hammer spur that they produced as well.
Other solutions besides the extension
If you still can't find an extension that works for your hammer spur, and still want to mount a scope on your Marlin lever gun, there are a few work around for this. You can mount forward of the action and above the forearm an extended eye relief scope. These scopes will eliminate the need for an extension overall as they put the eyepiece several inches forward. You can also move towards an offset scope mount that tuck the scope to the left or right of the centerline of the rifle. However these are sometimes flaky and often do not hold a proper zero. Another work-around is to buy extra high scope rings for your standard scope mount, and use a lace-on or slip-on cheek piece to move your eye-level to match the now elevated eyepiece.
Alternatively, you can always get a smaller thumb...