I have a couple of drop weight machines - the scales are both different.
On my Klip machine, the bar has two scales - one on the left of the bar, and one on the right. I.e. If you let the weight bar rest down, you can look down onto the left scale. My scale has two numbers, i.e. above the line is in lbs and below the line is in kgs. At the big numbers, i.e. 30, 40, 50 the marking on the line is a double line which for me - indicates that both weights are used.
Now, if I raise the bar and let it sit on the left above the clamps - there is another scale - this is for a single weight (so the additional weight held by a bolt should be removed). Again it shows lbs above the line and kgs below the line.
I agree that this was quite confusing originally as I did not have a manual. If in doubt, you are best to get a tension calibrator - even the spring ones will do because all you need to know is how to read the markings. Does this help you?
Ok, my old machine is very very old - it only has numbers for tennis, but then there is a printed sticker that shows what the corresponding tension would be for a single weight.
Also you can check with W&D Strings for info on the Pro's Pro Challenger 1.