Mike, this would be a good question for John. I'll see if I can get him to reply.
In the meantime, here's what I believe.
The axle was painted at the axle assembly plant (i.e. the Detroit Gear and Axle plant), but not filled until it got to the vehicle assembly plant (Norwood or Van Nuys). This leads me to believe that some kind of plug or cover may have been used to cover the fill hole when the axle was painted, and then the plug and tag (which came as a "unit" - P/N 3866433 - see below) installed after the axle was filled.
Of course, this is just a guess on my part, but it seems to me that if the "unit" were painted at the axle plant, it would be difficult at best to completely mask off the tag and I don't recall ever seeing any overspray on any posi tags.
Ed