It's pretty cut and dry; who else made KT88's in the 1960's?
Who actually manufactured what in the tube world was much more simple back in the Good Ol' Days of tubes.
I'm pretty sure all KT88's prior to the late 70's-early 80's and possibly later (I think GEC went away about 2000)were manufactured by M-OV (Marconi-Osram Valve Company), a subsidiary of the British company "GEC"; General Electric Company Ltd, not to be confused with the US General Electric.
These M-OV tubes were branded as Genelex, GEC, Genelex Gold Lion, but also were often private labelled for various other manufacturers/OEM users, including IEC/Mullard in the UK and Sunn in the US. I think they also screened them as Dynaco for a while.
I always thought that a "Mullard" branded KT88 was kind of ironic, but they do in fact exist.
I've got a pile of the Sunn labelled KT88's, and they are unquestionably GEC tubes.