This tube varied alot during it's production lifetime. Earlier versions look very similar to the RCA's since they also had the black coating inside the tube. Early versions also had cylindrical black plates like RCA's. Later ones produced during the 50's and 60's had flat gray colored plates and look more like their 6SN7 counterpart. The 6SL7WGT is the military version and usually will have more support than the commercial version.
More support usually equals less microphonic problems.
Some of them also had brown bases which are probably repackaged military 6SL7WGT's. Any of hese will certainly outlast and sound far better than ANY of the shit produced today. Like all of the other brands mentioned, these also vary somewhat in appearance and construction depending upon when they were made. Early ones have black cylindrical plates and bottom getters. The glass is clear.