I did a bit more research, and as I mentioned above, it appears that Ruger did manufacture a decocker varient of the P85 (but NOT the P85 Mk II, and ONLY in 1990). This is further substantiated in The Blue Book of Gun Values, p. 1796, and in the May 1990 issue of Shooting Times, p. 48 and p. 54.
I believe that in 1991 until the introduction of the P89 in 1992 that there was not a decocker varient catalogued, but I suspect that organizational orders would have been fulfulled.
The P89 was originally introduced as a decocker only, and it was only some time after its introduction that the P85 Mk II was phased out sometime in 1992, and the P89 then was available in all action varients (SA/DA, decocker, and DAO).
In addition to the revised safety that formally was incorporated into the production line on the P85 Mk II (and retrofitted to previous P85s), I believe that the P89 was differentiated from the P85 Mk II by incorporating the rear sight set screw (initiated for the XM10 trials), and some other subtle product/manufacturing improvements and refinments to the components (which continued throughout the P89's production)
The September 1991 issue of Handguns has a good article about the P89DC's introduction and characteristics on pp 54-61, and p. 85 by the late Dave Arnold.