So many people have gotten the bug to add disc brakes to their old mountain bikes lately, but they've run into a wall. The frame doesn't have the standard mounts for a disc brake caliper. (better known as 51mm I.S.) The 51mm International Standard mounts are necessary for the caliper, therefore A2Z components had a stroke of genius and developed an adapter that will sandwich the non-drive side dropout giving you a place to mount your disc caliper.
Here is a picture of a vertical dropout with no I.S. tabs:
Here's the A2Z DM-UNI brake adaptor:
The product installed:
A2Z Components website