I have 2 1200's and searched for DOB info for several years. If I remember correctly some Winchester records were destroyed in a fire. Best I could figure is the 12 gauge Skeet grade ser no L856XXX is late 60's / early 70's and the matching 20 gauge Skeet grade ser no 177XXX is early 60's. Yup! A matched set. Looked for nearly 45 years to find the 20 and have won my club's top gun sub gauge match(s) with it several times. That's as close as I could figger on the DOB's and it's been so long ago I forgot how I came to that decision. Good luck.
I recently acquired a 1200 and perused Google looking for general information. I forgot which forum it was (it was Winchester-centric), but it had a good thread concerning serial #'s and dating.
The info there stated that Winchester began with #100,000 when production began in '64 and they produced approx. 75k - 80k guns per year. Mine is #250xxx, so that should put it late '65 - early '66 production, and the OP's #218xxx should be a solid 1965 gun. They added the L prefix after the GCA in '68.