Be glad you didn't get it for even close to that price. That's WAY too much money for a model 18, even with $200-$300 worth of stocks!
Those Combat's came standard on the 14-6, 16-6, 17-6 and 617, which had full lugs. You could get the 16-6, 17-6 and 617 with either a 4" or a 6" barrel. I don't think they offered a 4" 14-6.
I've never seen or heard of an 18-6, so the Combat stocks were likely added at some point. The were sold as accessories and more than a few were taken off guns.