Hidden? handprinted numbers added to my data plate

[Mariposa Mike]

Just sandblasted my data plate today to clean it up for a repaint and found handprinted, indelible numbers, likely factory, added to the plate maybe before painting. They are separated pretty far apart and has anyone else found something like this? Number 3 and then 12 with another digit after that but not clear. Might be a 1. I was thinking maybe an inspectors mark too. The marks resemble a marker I use to mark metal. Is that enough marks in one sentence? Anyone have an idea about these? The data plate was previously decoded before removing the paint.

 

[Mariposa Mike]

I forgot to add the picture.

 

[YY1]

Inspectors?

 

[Mariposa Mike]

Also, were these usually painted along with the car or bare metal? Seems every one I have seen is painted but the cars had been repainted too.