> I'm not sure how to read the osccal value, but I've done a read on the
> four
> eight-pin devices I've got in stock and they all have a value written into
> the top address.  Is this the osccal value?
>
yes, although this is actually an instruction: movlw 0x70. the
calibration value is 70 in this case.

> On the basis that it might be, here are the values:
>
> 12C509A: C70
> 12C509A: C74
>
> 12C508A: C70
> 12C508A: C74
>
Adding to the statisctical samples:

I have two windowed 12CE518 devices on my desk,
calibration values of 0x8C and 0x94.

I have 3 OTP 12CE518 devices with calibration values
of 0x20, 0x30, and 0x40

(yes, i know they are slightly different devices, but
they have a much larger spread than the ones
mentioned above)