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For example, it would tell, in 1964, the domestic flat rate for a letter in Bolivia was X bs
This would be easily discernible by the denomination of the stamps issued at the time. The most common denomination would be the letter rate.
As for (as an example) the airmail rate in 1956 for a letter from Bolivia to Fiji (or any country you like), you'd have to be a specialist in the postal history of Bolivia.
Postal history is such a massive field that you just can't be a specialist in every country. To bring all that information together would be a multi-volume production. The lack of demand would kill any prospective financial undertaking.