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So can a boxer legally do a spinning backfist or not?

Well, either you or kung lek are full of sh1t, cos what you both just said is mutually exclusive. ;)

KL reckons you can hit with any part of the glove, which would include the back. You say no.
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So if a boxer pulled off a spinning backfist he wouldn't be penalised for it?

"Only jab, uppercut, cross and hook"?

Think about the combos available. There's no "only" about it! ;)