When you say "very good" condition, it sounds to me like maybe it's just a shooter. Therefore I'd say $600 is high. That gun in excellent condition is worth $600 easy. The stocks look nice. Do they number to the gun? That's a major asset if they do.
I believe that a 19-4's grips wouldn't number to the gun, as it would be a late '70s vintage, they had stopped matching the grips to the gun in the '60's. I could very well be wrong though.
I think that is a ~$600 gun with $600 being high, I agree there, but I'd not be afraid to pay $600. The 6" Model 19 is such a sweet shooter.
A while back I saw a near perfect 6" 19-2 with perfect Diamond targets, I drooled over it, and was very close to saying I'll take it, but it would have been a safe queen. I have two safe queens, so no more for me, only guns I will shoot.
So I passed and got a well used but nice 686 that I shoot.
So like you, I would recommend offering ~$550, and see if they take it.
That will be fun to shoot, and with a little Mothers and Ren Wax, it will clean up real nicely.