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The way the manufacturer sized models for the North American market at this time the 21" nominal size was 54cm c-t-t. Nothing in the 55,56,57cm c-t-t size was offered until one reached the very top models. The next size up was 23" nominal which measured 58cm c-t-t.
Only know because ran into this back when these were current as my size is ~56-57cm c-t-t.
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