Sorry for the somewhat late reply.
The Kenko I have is the Kenko Teleplus PRO 300 DGX 1.4x AF Teleconverter. The image is not 100% the same one as I have, but very similar. It has 8 pins on the other side.
IMHO it's not usable. AF is ok (on a D500) but IQ is lousy, but I tried only for a few images.
First one is at about 5m, the second about 25m. Both very soft and hazy (it's not the lens nor the camera
)
I did not tried the converter with other lenses yet.
Bot images are cropped to about 1800px
@Bjørn - I take it with me the next time we meet