I have the same Bird's Head Vaquero as seen in the OP. Love everything about it except for the awful faux case hardening. It would have been 100% better being blued only.
I have a birdshead New Vaquero in .44 magnum. I have no idea how many were made. I think they were made as a run for Talo. The key to tell the heavy frame from the lighter one is the serial number prefix. A 2 digit prefix is heavy frame, a 3 digit prefix is the lighter frame. I believe this only applies to the Vaquero.