Kirk,
GPC(Numrich) is notorious for sending "something close" to what you ordered. Ebay, is a crap shoot as items are often mislabeled. The firing pin you received looks like it is for their Hammer model. GPC are good about working from drawings or pictures to find the correct part. I would contact them by email an try to identify your pin from the drawings or pictures.
Can you give us a measurements: Frame width across the flats. Side plate length and height.
If these match 12ga dimensions I will remove the firing pins from one of my Meridens and send you the specs.
John
