Serial number?? Which one?

[IdleOne]

Hi,
I own a PA120 head.
On the back of the chassis it says serial no: PA006, but on the back of the removable panel that covers the tubes it says OA2313 which one of these constitutes the serial number I can use to try to date the thing. I mean it seems like a 1970-71, it's a pics-only faceplate design and the pots are on an orange background which I understand to be a good way to tell that this thing is 70' 71'. Anybody have an idea how old this thing is? I play guitar through it and it sounds great... is that a bad thing? I mean it must have been designed for mics right??? Sorry to bombard you with all these inquiries...

IdleOne