looks to me like the original plates are burnt Kiwi!!
I think they might have got a bit hot, but it’s not clear if that caused the slippage, or whether the slippage caused the hotness 😆.
I actually bought some new unbranded friction plates before the Barnett ones that also worked fine without excessive spring pressure, but they turned out to be made of cheese and the tangs virtually disappeared after 500 miles. See before and after pics. At least they were nearer the correct thickness, unlike the Surflex (and Barnett plates for that matter).
Although my experience with various duff plates is with a B44 it does highlight the urban myths that can arise due to the existence of poor aftermarket parts. The B44 clutch is pretty universally maligned as being not up to the job and heavy to use, with expensive aftermarket pressure plates, methods of fitting extra plates etc commonly used to fix the issues, whereas on my bike it was simply a matter of finding a good set of friction plate (that lasted). Oh, and fitting the correct length spring cups.