If you are looking for a reliable hunting rifle, you probably won't be disappointed. I have a left-handed 788 in 6mm Rem. and it shoots very well. I'm happy with it.
I agree that it is probably an aftermarket stock. I've never seen a 788 with a synthetic stock.
As for what it's worth, it's only worth what you are willing to pay for it. If you are looking to turn it over and make a few bucks you'd probably have to pick it up for $400-$450.
Jerry
