The 6SL7GT is a medium impedance double triode with high gain and superseded the 6SC7. It featured separate cathodes and was designed for use as AF voltage amplifiers, phase splitters and where required as a cascade amplifier of very high gain.
It was also specified for US Government projects with a procurement code of VT-229.
The Sylvania name and VT-229.
The plastic shrink fit plastic band is probably original.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 67 mm tall.
References: Datasheet, 1043 & 1040. Type 6SL7GT was first introduced in 1942.