I wouldn't take $700 for my 629-2 with the same barrel length. And mine was drilled and tapped at the factory. I happened to take the rear sight off and found the holes. No screw has ever been in them either. That much was clear. It has no marks on the metal anywhere from where a screw would have been so there's little chance it's had any screws in it. I could be wrong but I don't think so.
And this is just my 2 cents but a Rock River AR probably hasn't held it's original value very well. There are several entry level AR's that IMO are better than the Rock River models that were in that market segment a while back. S&W is one. I know my experience with Rock River AR's wasn't great. I've shot several at the range and they all invariably ended up jamming. I'm not saying yours does but people probably worry it will. A new M&P Sport II is selling for $500 in many places so I wouldn't expect an older Rock River to bring a lot more money than that. Times have changed in the AR world. That's likely why you haven't seen cash offers for your rifle. But that's my opinion.
Just as an example of my thinking people see RR AR's selling like this:
Rock River LAR-15 Mid Length A4 Carbine 5.56 16" Barrel Optic Ready 30 Rd Mag - Impact Guns
I don't know anything about your rifle but people are going to know they can get a new RR rifle for less money than your used one and given the fact that some of them weren't real reliable and that S&W, Savage, Ruger and Springfield all now make entry level AR's I see the value of your rifle being impacted by all this. Plus with no real threat of gun control for now the prices have gone too.
In short I'd make that swap in a second. You may have a great AR but people wonder why others are selling a gun too. I do anyway. I wonder if I'm buying someone else's problems. IMO S&W revolvers have a much better reputation for quality than RR AR's. You could have the best RR rifle ever made but convincing people of that is another story. I once saw a RR at the range that two guys were trying to shoot. I say trying because they would shoot it maybe 3-4 times then they would work on it for 15 minutes trying to clear a jam of some sort. They were working hard to get that rifle running again. Then the same thing - 3 or 4 shots and another jam. They must have fought with that RR for 90 minutes before deciding to take it home and fix it. Again your RR may be nothing like that but I saw several RR's at the range and pretty much all of them had issues. I can't be the only one that saw such things.