Question on Sun tach dates

[1967Z28]

Does anyone here have any idea how to decipher some of the less obvious dates on the back of old Sun tachometers? I have a ST-601 with a "526" in orange ink stamped on the back and an SST-802 with "1505" stamped in black ink on the back. Any help is greatly appreciated.

-Jon

P.S. Attached is a pic of the back of the ST-601

[1967Z28]

Here is a pic of the back of the SST-802. If you need to see the front of the tachs, I can post those pics also.

-Jon