Warmest of welcomes from Connecticut, hope you enjoy the fellowship and are active in the forum. Great people and great guns.
I am a newbie to pistols but I am sure someone more knowledgeable than I will post a answer to your question, and again welcome.
I am pretty sure that they will, but would not swear to it. I know that my P89 would interchange with my P95, but I do think Ruger changed the magazines slightly once over the years. If you go to Rugers website (I am at work and the blasted thing will not allow me to go there, although it lets me go here!) and look at the magazines for sale they will have magazines for the P89 up to a certain serial # and then magazines for the serial #s after that. So, the P85 being an even older version of the P89, they may not work. This is all just a guess on my part. I will shut up now and let some one who knows the awnser to the question chime in!
OK, here's the scoop! You have about a 90% chance that the magazines will interchange. There was point when they raised the magazine .040" to allow for a more in-line feeding because of some of the new large hole hollowpoints that were being put on the market. The problem is that some of the mags won't go up far enough to lock in place. They bottom out on the mag floorplate before reaching the lock. If the mag will lock into place properly, they should work without a problem. However, as with any mag that was not shipped with the gun, make sure you do a bit of testing for reliability before you use it, especially if it's going to be used in a carry gun.