Your Mod 659 does seem unique. It may not be a true RSR transistional model, but don't forget S&W never threw away older parts during a change. You have what looks like a Mod 5906 frame marked 659 with a dove tailed front site and the older style ambi safety. To the right collector this combination will be worth extra money. Either way you have a nice pistol there in great shape.
The best thing to remember about S&W is that they never always did everything the same way. You have one of those unique guns that aren't normal production models.

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J.D. Roy