I know this is a bit late, but here it goes.
I found a chart in another forum, the OP claimed its Remington's serial #'s for their current manufactured rifles.
B - jan
L - feb
A - march
C - april
K - may
P - June
O - July
W - aug
D - sept
E - oct
R - nov
X - dec
E - 2010
F - 2011
G - 2012
H - 2013
I - 2014
J - 2015
K - 2016
Most newer models start with "MR," then the following two letters indicate the month and year. ie serial # starting with "MRPJ" would be June of 2015. I think it's also safe to assume the letters "L" and "M" would stand for the years 2017 and 2018.
If you know the SN of a rifle and want to confirm its manufactured date, I'd say just call Marlin and they can confirm it if you're ever in doubt. Hope this helps.
I've been looking at a 336BL as well. Personally, I would not trust anything made before 2015 since Remington took over.. based on the reviews I've seen, it's been a mixed bag up until 2015, to which I'm still skeptical of the 336's quality.
Nice looking rifle, it's really too bad Remington's takeover engraved doubt into the Marlin name.