pplinker,
I was referring to the lead slug when I said it was Most likely 12ga.
To get a more accurate "guess" of your fired slug a dial caliper reading of the
diameter of it is needed.
12ga. = 0.729"
14ga. = 0.693"
16ga. = 0.615"
Your slug is a lot newer that that No. 14 headstamp.
Winchester New Rival headstamps were used between 1897-1929 with a few variations. The only other way to narrow down that date range is by the color of the paper cartridge, witch you no longer have with yours.