Here's how to do it easily. Push the half-moon-shape retainer back with a Vise-Grip plier and clamp it to the guide rod - held back far enough to file off the crimped-in ridges. If you then are going to change the spring, be careful where and how you release the Vise-Grip in order to avoid losing parts or being hit by them as they fly off.
This is a relatively easy job to do - actually, easier than disassembling the MkII for the first time unassisted!!