The safety mod
With the original P85 pistols, there was a chance of the firing breaking in a certain way, and when you decocked it, it would fire a cartridge in the chamber. The MK II modification corrected that problem.
If the model on the slide says RUGER P85 MK II or there is a MK II on the decocker lever the safety mod has been done, if it does NOT have either of those marks, the pistol can be sent to Ruger for the modification free of charge. You do have to pay postage, but when they return the pistol to you, they will include a magazine to reimburse your postage cost.