The type 6SG7 was designed for wideband RF and IF amplification and has a relatively high gm value to ensure a good gain-bandwidth product. It was adopted by the American Government and coded VT-211 as is shown on the can.
The valve has dual cathode lead-outs to minimise the risk of self-oscillation due to parasitic coupling between input and output circuits caused by common impedance in series with the cathode.
The 6SG7 and 6SH7 are essentially similar except for grid pitch. Type 6SH7 is a 'straight' pentode while type 6SG7 has a variable-pitch grid, giving it a vari-μ characteristic, and would be suitable for the IF strip of radar receivers incorporating 'swept gain'.
Made in USA.
Most unusual to have Sub-Standard printed on the can.
The RCA logo.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 50 mm tall.
References: Data-sheet, 1040 & 1043. Type 6SG7 was first introduced in 1940.