The picture at left shows a 6CB6A rebranded as "6JK6" by International Components Corporation to the left of a real 6JK6. The third tube is a correctly marked 6CB6A. The second picture shows the same tubes rotated to show the internal structure. Note that the real 6JK6 has a sold plate all the way around, but you can see through the 6CB6A tubes.
The characteristics of the 6CB6A and 6JK6 are actually similar, except the 6JK6 has about twice the plate resistance of the 6CB6A. This makes 6CB6As unusable for the only important 6JK6 application: A Marantz 10B tuner.