The gun in the condition you describe, with original box, doc and tools, and all the original parts (stocks?)....I would place the value range between $475 and $575.
Now, you also mention..."
an extra set of wood grips included. They are a darker wood, no checkering, with S&W medallions". If these are smooth rosewood targets, and they can be lettered to this gun, I would add $50 to $75 to my price range above. In my opinon, these stocks may be the key to the real value of this gun! If you buy this gun, make sure to hang on to these smooth targets until you determine their history!!
So, with this considered, I would go in and offer $525-$550 cash and see if they bite. If so, then you'd have a very nice model 66 for a more than acceptable price. Especially if the other stocks are rosewood and they letter to the gun!!
This.
Otherwise, there are some real low-ballers in this thread.
It's hard to tell from one picture, but based on it and OP's description, cosmetically this 66 is between very good and excellent by NRA moderns firearm standards, leaning toward excellent. If the same is true mechanically (and OP must run the gun through several checks to confirm), this is a desirable item.
If the box is original and numbered to the gun, with tools and papers you're approaching collector's grade 66; extra stocks and original trigger and hammer only enhance that, particularly if as MacA pointed out the extra grips are smooth rosewood targets from the factory and matched to the gun.
If indeed everything is as it seems, $595 is not a bad price -- if you look at what similar items are selling for on the open market, it's in fact a pretty good one. (If you have an account at Gunbroker, you can see what auctions are actually selling for, which provides a good window into what market prices are).
Bottom line: confirm cosmetic and mechanical condition, the box and spare items, and if it is all to your satisfaction, make an offer. If you walk out the door with everything for $595 on the nose, I'd still say you did fine; if for less, all the better.
As for the substituted hammer and trigger -- they're fine if from the factory, and either dropped straight in or, if a little fitting was required, was done properly and professionally.
Same goes if you want to reinstall the originals. It's fairly straight forward if you do some basic research on revolver internals, or you can ask a gunsmith to do it (should be very inexpensive). If the store has a gunsmith on site, you could ask them to do it as part of the sale.
It's always difficult to give advice online, where only pictures and descriptions are available to judge by, but again, if everything aforementioned checks out, and this particular gun speaks to you, I say buy it.
