It was sold to be used with the Joystick antenna. Which was widely advertised in the 1960's.
Page 519 of Shortwave Magazine November 1963 has an advertisement for the aerial and the optional Joymatch ATU which could be purchased for an extra 10 shillings.
You can see the advertisement in Short Wave Magazine on the AmericanRadioHistory site. (Page 519)
https://www.americanradiohistory.com...WM-1963-11.pdf
John