Mfg date ...vs ship date ...could be awhile.../ I think the mfg date for the serial number was late in 1989 ...so its easy to see how it could have stayed in the assembly area for 6 months or longer in those days ( before Just in Time inventory went nuts )....
maybe the original grips were damaged ...even the dealer might have changed them for some reason...
( guys like me - have a whole 20gallon storage bin full of extra S&W revolver grips...)...
686's in 4" and 6" in my area now, used, but in excellent condition are valued at around $ 600 .....so in any event, you have a nice gun / I like the 686's ...and model 19's, and 27's, and 28's...in .357 mag..( and have more than a few ) ...
..../ .357 mag is my favorite caliber.