I believe it is an independent grading service... I am unsure. I got these slabbed coins off Listia - I was expecting PCGS, ANACS, or NGS... but I got SGS.
besides these coins - I have only ever owned 1 other slabbed coin. It was a 1934S Mercury in PERFECT shape - Full split bands... I shipped it off to PCGS and it came back slabbed but it had a HUGE scratch on the obv... no full split bands... NOT MY COIN. THAT was back in the 80's... I have had nothing to do with slabbed coins since - but it appears they are all the rage these days. I personally like to hold the coin in my hands - no white cotton gloves either... but... I have been told I am wrong.