Hi Chris, they have three little half moon shapes on the underside edge. A lot of the modern ones also have a small circle around the base of the peg, which isn't present on the original. Post up some pics
Although this is true about the circle around the peg on some repro ones, the ones Iain sells don't have this and look the same as the originals. Fortunately I have some originals and recently had a close look at these compared to the repro ones to see if I can tell them apart. From what I can see the placement of the 3 half moon shapes differs from the repro ones.
Edit - James contacted Iain and his stands are the ones with the circle around the peg so the other ones that look very close to the original are from somewhere else. Not trying to dirty your name Iain, honest
One of the half moon shapes should be directly beneath the peg (12 o clock) with the other 2 at roughly 5 and 7 o clock on the originals. The repros have the half moons opposite with the one at 12 o clock on the originals being at 6 o clock. Hope that makes sense but I've looked at the set you sent me Cazza against my originals and also an Action Force original and the half moons all match. If anyone has repro ones and it's different from what I've just mentioned then please pass the info on here.
All of the above is for the clear ones and I haven't looked at the yellow ones so can't comment on them at this time. Also all the repro clear ones are clearer than the originals (just by a bit), as they must be using slightly different plastic.