These Groove Tubes Gold Series GT-6L6-S Matched Power Tubes are Russian made, same as the 5881, but matched in the GT system. These are a very sturdy and reliable tube, with slightly less power and a more aggressive edge when pushed into distortion.
These are the tubes that are used by Fender in amps that use 6L6 tubes and DO NOT come from their Custom Shop. These are used in the Hot Rod series, Blues DeVille series, Pro Tube series, BF reissue series.
Low, medium, and high style descriptions refer to break up modes of the tube for an overdriven guitar signal. High-rated tubes break up slow and have the highest amount of headroom, while low-rated tubes break up easier and have the least headroom, yielding a heavier, harder distortion. Medium-rated tubes fall in between.