Palindrome wrote:

I'm not sure exactly what that band is for. It could be there to reduce
radiation of the transformer's eddy currents

That's exactly what it's there for. Actually it uses eddy currents in the copper
band to reduce the transformer's stray magnetic field.


and thus satisfy emission regulations.

Or improve subjective performance in some way (far more likely imho).


It could be there as part of the heat transfer mechanism.

No.


However, they wouldn't have put it there without good reason and the
mechanical design of the transformer is key to its electronic
performance - so I would try to get it back in position and re-join it,
idealy across the complete width, to form a complete electrical loop -
if that was how it looked to be intended to be.

I agree.

Graham