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How does the Mercury part numbering system work? There seems to be a 5 digit number; followed by an "A"; and, then followed by another number. That is, there'll be part numbers

12345A1, 12345A2, 12345A3, etc.

The parts appear to be similar. What is their form, fit, and function compatibility? I'm looking at a model year 2000 engine. Some of the part numbers are now up to "A14." Can I assume that an "A14" revision level will fit and work where my engine calls out an "A1" or an "A2" part number? Are they backwards compatible?