I don't know which Gibson Lab series were made by Garnet, only that some of the were pretty well regarded. Ty Tabor and B.B. King were Lab users at one time.
It was some models (don't recall exactly which) in the Lab Series 2 (the well regarded ones are the Moog-designed original Lab Series - mostly the L5 and 7 - same amp, but 2x12 vs 4x10).
You can tell it's a Garnet (despite the branding on the front) if it has a sticker like this ont he rear of the chassis (minus the Garnet logo except on models branded as Garnets):
I was under the impression that the amp was built at the Standel factory. Gibson quit amp production around this time at Kalamazoo. CMI had purchased the holdings of the Standel Co. A book of mine states that the "G" Series amps were built there, but no mention of the "Batwing models or the LP-12.
Yes, that's what I thought too and it would make sense - the SS preamp head with the power amp in the speaker cab was a Standel sort of thing to do.