Winchester model 1200 20 gauge – serial number L507905.

Does anyone know an approximate date of manufacture for this shotgun?

Also, an owners manual or exploded view PDF, something along those lines to aid me in breakdown, cleaning, and reassembly?

This shotgun was a birthday gift 2 days ago but was in disrepair upon receipt. I have broken it down and found that the firing pin has somehow disengaged with the bolt and the spring was rolling around loose inside of the trigger housing. I would just like a reference to ensure I reassemble it correctly.

Thank you in advance!