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No, the base (numeric) part number is different when the plug and/or voltage rating is different. The letter is a product revision. The 82059A wasn't suitable for later calculators. They added a slot in the plug and a keying piece in the socket to prevent the 82059A from being plugged into later calculators.
I'm not sure about the difference between the B and D; it's possible that it is just a manufacturing change with no significance to end users. Check the manual for a specific calculator to see which model is recommended; later revisions should work fine. In other words, if the manual calls for an 82059B, and 82059D will be fine, but the reverse may not be true.