Hi Chris
I have one of these Celestron Counterweights - 17 lbs Designed to fit 19mm counterweight shafts.
https://www.sirius-optics.com.au/mou...ht-17-lbs.html
My photo shows it with a Vixen SP/SP-DX CW shaft, which I believe is the same as the GP/GP-DX CW shaft.
Cheers
Dennis
EDIT: On our uncalibrated bathroom scales it weighs is at just under 8kgs.