I took the liberty of emailing Gibson, (only a couple of days ago!) Here's their reply - seems even they are confused about their numbering system.

"Hi James,



Thank you for the email.



According to the serial number this appears to be either a 1974 or 1975 SG Special. This guitar came with Gibson mini-humbuckers. It was available in cherry and natural only. The serial numbers through these years will not allow us to specify 74 or 75 due to some numbers duplicating themselves.



............


Regards,

David Osborne

Gibson Musical Instruments".

So now we (almost) know.

All the Best.