6SL7 is an octal tube with dual high-mu triodes. Higher mu makes this tube desirable for use in voltage amplification stages of audiophile tubes.
There are several variations of 6SL7 available. 5691 is the industrial long life version with extra mica supports for lower microphonics. VT-229 and 6SL7WGT variants are military spec versions. Usually VT-229 is from much earlier era than WGT.
6SU7GTY/6188 is a sleeper of 6SL7. 6188 is a special instrumentation 6SL7 made for industrial use. 6188 has multi-layered cathode, long plates and factory matched triode sections.