The Mountaineer and UL are cut for a longer breech plug(deeper into the barrel). But the Mountaineer, UL, and DISCs all use the same bolt(not counting the different finishes). The difference is inside the particular barrel. I have an UltraLight barrel and breech plug with a Mountaineer bolt that was in my Original DISC, and a laminate stock that came from my 54-120 carbine. The only special fitting required was making room in the stock for the bigger forked recoil lug and bedding.
If you put the shorter DISC plug into a Mountaineer or UL barrel it will go in much too far. Likewise if you put the UL Mountaineer plug into a DISC you would never be able to close the bolt.