The old 'On-One' version had a rolled seam at each end of the tube to hold them up. I think they are these ones now. https://www.planetx.co.uk/i/q/CLCAMSAW/carnac-seamless-merino-armwarmers
Edit: https://www.planetx.co.uk/i/q/CLOOMSAW/on-one-seamless-merino-armwarmers
The on-one link says seamless so I guess they both are.
Can measure the bicep bit when I get home if you want.
@hippy started
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The old 'On-One' version had a rolled seam at each end of the tube to hold them up.
I think they are these ones now.
https://www.planetx.co.uk/i/q/CLCAMSAW/carnac-seamless-merino-armwarmers
Edit: https://www.planetx.co.uk/i/q/CLOOMSAW/on-one-seamless-merino-armwarmers