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Trying to get an accurate measurement of 20 year old wood is almost impossible, expansion/contraction/being down and dry for 20 years/or being down and wet makes it almost impossible.
You are correct to put down 5/8" as a sub floor and even more correct to use thinset to make up any difference in heights.
No 19/32 20 years ago is typically the same as 19/32 today, but for the previously mentioned reasons there will be subtle differences between old and new this is really not all that uncommon.
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