Hi Maarten,
yes, your right the Radiomuseum simply states that TFK was a brand making and marketing the A66-120X.
Same goes for the A56-120X:
https://www.radiomuseum.org/tubes/tube_a56-120x.html
I've always believed Mullard took on the development of the "square corners" A56-120X, but I could be wrong.
DFWB.