Is there some reason to think that the Owensboro plant under GE ownership (as indicated by the "188-5") was producing two distinct 6SN7GTs in 1950 (as indicated by the "0-35") -- that is, a "GE" version and a "Ken-Rad" version? I think that this is highly unlikely.
I have no facts but I agree: quite unlikely.
P.S. Go Iceland!