Using this handy chart you can see when your SPAS 12 was produced!  As per the original post on Shotgunworld.com (link here):

"All firearms made in Italy are required by law to be tested by the Government Proof House, which marks the firearm with several symbols, one of which is a code identifying the year in which the firearm was proofed. "

To find the code on your SPAS 12, turn the gun upside down and look where the magazine tube connects to the receiver.  You should see a smattering of markings and symbols.  Standing off by themselves in that group will be two letters, these are your identifying markings to see when your SPAS 12 was made!