Span classnews_dtapr 02 2014spannbsp018332serial numbers located on the lower tang under the lever these are squarebolts and are not to be confused with later roundbolt glenfield 36 336 roundbolt action late 1945 production resumed on limited basis using c prefix and leftover parts some guns will have prewwii model 1936 parts mixed with post wwii model 36 parts.
Span classnews_dtjan 16 2009spannbsp018332the early model 1895s had the b serial number prefix up to 1977 when they went to the other system of serializing where you subtract the first two digits from 100 the jm you mention is the proof mark applied to all marlin rifles.
marlin model 336 and models 39a and 39m manufactured from 1948 to 1968 a one or two letter code was used to designate the year of manufacture all marlins except lever action manufactured from 19621968.
serial number u is 1961 the following table can be used to date the year of manufacture of a marlin 336 it is also valid for determining most other marlin firearm build dates from 1946 to the present.
Your serial number indicates that your marlin model 336 was made in the year 1971.
Span classnews_dtsep 03 2010spannbsp018332from the marlin 336 wiki the following table can be used to date the year of manufacture of a marlin 336 it is also valid for determining most other marlin firearm build dates from 1946 to the present.
Span classnews_dtdec 08 2011spannbsp018332remington uses date codes on the barrel i have no idea on how to decipher those remington has continued to use marlins old north haven address on the guns made at ilion ny even after shutting down the north haven factory last year.
Span classnews_dtmar 08 2017spannbsp018332how to find the date that your marlin rifle was manufactured based off of the serial number httpwwwmarlinownerscomforummar.