How common is factory Porsche overstamping on parts?

[911split]

Just how common is factory Porsche overstamping on parts?

Here's a few pics of an example from a spare early 911 Speedo of mine. It's interesting to see 3 different date stampings on the back of the speedo.

1.) "7 66" is stamped into the metal of the speedo housing
2.) "1 68" is rubber stamped over the metal date stamp mentioned above
3.) "2 68" is ink stamped within a triangularish ink shape nearby

Do they just indicate different approval stampings as the speedo made its way down the assembly line? Is it an anomaly, very common, or all they all like that?

Thanks,
John.