IBM 5100 exact key dimension

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Does anyone have a "loose keyboard" or a loose key from an IBM 5100 or similar (5110/5120)?

We were trying to find the exact dimension of a single "normal key" (e.g. A-Z, 1-9). I have a keyboard from those systems, but with the keyboard still in the system I can't get clear to the base of the key to measure - I estimate it is 20mm square, but was just wondering if anyone had an actual physical loose key to verify.

Thanks!