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Thanks -- I just saw that you listed more details further down in the instructions. I was initially looking at the summary.

I found http://www.mcmaster.com/#9657k306/ which matches the desired dimensions better, but I suspect what really matters is the spring rate which is a result of the diameter, length, number of windings, wire diameter, and metal type (I've been dealing with springs a lot lately while 3d printing a Curta calculator -- wudev.digitaltorque.com if you're interested). That one on McMaster-Carr is rated for 8.2 lb/in which is around 1.46 kg/cm. Does that sound right?