Butt Plate Markings

[Garnett]

A friend has shared with me information about the butt plate on his M219 rifle.  He has a Second Model Utica.  The butt plate is the proper plate for this model with NO re-enforcing rib between the screw holes.  The inside of the plate is marked with a single digit "7", which is not unusual.  However, instead of other digits or letters, or symbols (like a Spade), it has what appears to be a serial number......23122.  Does anyone have a similarly marked butt plate?  I find this marking most unusual.