[turretslug]

Supposedly, the 'W' stands for War (!), it seems to have been a post-WW2/'50s designation applied to quite a few valves supplied for US military use- 6L6, 6AL5, 6AK5, 6BA6, 6BE6 etc. often turn up with a 'W' suffix. I think that it often means a ruggedised version with detail production changes to resist shocks and vibration, rather than noticeably improved electronic performance. The 6SK7, a late-'30s modest-mu IO pentode, was getting a bit old hat in the post-war era, but this version probably illustrates that there was still a great deal of legacy electronics in the US inventory.