K291xxx would have shipped in 1957. The stocks you describe are called relieved target stocks. Target stocks were introduced as an option for the K-frame models in 1950. They came either non-relieved (no football cut-out) or relieved (with the football). If your gun has the full target hammer rather than the standard 3/8" speed hammer, it may have shipped with the hammer and stocks as original equipment. Sounds like the gun was not much used, so I suppose that is likely to be the case.
$800 sounds a little high to me, but not too much; it's probably at the high end of the proper price range for this gun. Several times I have paid next year's price for a gun I wanted today, and I have never lost sleep over doing so. I figure it is like acquisition insurance; I pay a premium to make sure that a gun I want goes to me, and not someone else.
As an aside, Coke grips are the target stocks for S&W's largest frame revolvers that have a pronounced palm swell on either side. The K-frame target stocks are good stocks in their own right, but don't have the unmistakeable coke-bottle profile you see on the N-frame stocks when you look at them from behind.
Pictures? We love pictures.
EDITED TO ADD: Target stocks were usually not numbered to the gun they were mounted on. Since they enclose the frame rather than matching its profile, there was no need to do precision fitting to a particular unit. And don't be in a rush to get some period diamond magnas unless you just want to have them. I prefer the early K-target stocks on my K-Masterpiece revolvers. I won't take off magnas that are numbered to the gun, but if I get a gun with some other number on the stocks, I have no hesitation about digging up some appropriate target stocks and putting them on.
And I would buy a high-condition older revolver over the closest modern equivalent any day of the week.