Borg's Headband, just responding to your question, but perhaps BF knows more. That's interesting isn't it? Well, I've read his contract with Bancroft expired in 1979, so we know he used Donnays after that (even in the US).
Look at this webpage I found:
Borg used a Slazenger Challenge Number One from 1973, and then Donnays of course. From about 1976 until 1979
, he had a separate contract with the American company, Bancroft.
See this comment that may shed light on what you asked:
"Borg continued to use the Bancroft Borg in Australia. La AKAI Cup in November 1982 was played by him - and won, outperforming the then NO 2 and 3 McEnroe and Lendl, Connors was No. 1 back - with a completely black Bancroft.'s Versatility.."