A bit of further playing with it suggests the thread is a bit knackered. One bar screws up properly, the other one tightens and then just continues to spin and never fully tightens up. Will email Helson and see what they have to say.
It probably won't be economical to buy a replacement from Helson because of postage costs.
You can buy them elsewhere though, some Bell & Ross and some Panerai models use the same screw bars.
@Stonehedge started
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A bit of further playing with it suggests the thread is a bit knackered. One bar screws up properly, the other one tightens and then just continues to spin and never fully tightens up. Will email Helson and see what they have to say.