When Winchester reintroduced th model 12 back into regular production in 1972 they added a Y prefix to the serial no. The gun control act of 1968 required all gun manufacturers to use letter codes....
A very worn field grade gun (but still in workable condition) might get you $350 bucks. From that point on condition, guage, and grade mean everything. I just visited a web sight that had M# 12...