I have a Marlin Model 31 rifle that I got from my grandfather but I can't seem to find any information on it. The only identifying marks are as follows:
It's bolt-action with a light brown wood stock and is tube-fed.
I'm considering having a gunsmith thread the end of the barrel and attach a scope mount to the rifle, but don't want to ruin it's value if it is a rare model.
The reason you cannot find any information on a Marlin Model 31, bolt action, .22 caliber rifle is because Marlin did not make one. The Marlin Model 31 was a hammerless, pump action shotgun made between 1911 and 1923. Most likely, what you have is a Marlin Model 81, and the model number stamp is a light stroke stamp that makes the 8 look like a 3. The Marlin Model 81 was made in various styles from 1939 to 1971.
Since your barrel marking does not mention the patented Micro-Groove rifling used on all Marlin .22 caliber rifles since 1954, your gun may have been made prior to that.
The thing about older guns is that they are no longer made, and when changed from the original configuration, there is one less of the older, original gun out there for future generations of collectors that may want to collect, original .22 caliber, bolt action Marlin rifles.
Why not buy a new .22 caliber rifle to have a scope mounted on, and use that, and save your grandfather's gun back for future grandchildren to enjoy as the original gun that their grandfather used.
Due to the increasing cost of ammunition, there will be no warning shot!
The growing federal deficit = generational slavery to the national debt.
