Just got a 686 2.5" and it came with S&W wood combat grips. One side is darker than the other. On the inside of the grips, written in pen, is the number 974 on the left side grip, and 874 on the right side grip. Serial # on the gun ends in 784.

The numbers on the grip panels don't have anything to do with the serial number of your gun. The numbers are to keep "matched" panels together. Looks like someone misread the number and erroneously put non-matching panels on your gun. (Or, it could have happened outside the factory.)

Just my guess.

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Lorin