Thanks Malone, it's the test for fit part l don't get. Why is that a meaning for offer up. That can only mean foregoing something like a gift to the gods or something can't it?
Much head scratching going on here lol.
Rozbogs, under 'Offer Up', Chambers has 'in eg joinery, to position on a trial basis, in order to test for size and suitability before fixing' - so it seems to work, just!