[mattr47]

My Dad was a morse intercept operator for the US Army, mid 60s, stationed in Northern Japan. He has stories of them naming all the Soviet morse operators by the way the tapped.

[Rodeoclash]

Cryptonomicon mentions this as well. Each operator having a particular "fist" that was unique to them.

[Aloha]

It's still true now in ham radio, even for those using a bug (vibroplex keyer) or even an electronic keyer, will still leak some of their natural fist thru.