You don't need that to operate the press, that's why they are so hard to find that and they're a PIA to use. Most Lee users use an 'off the press priming system' I used a Lee hand primer for years until I wore it out. The one I have now is made by RCDS and is sooo much nicer then the Lee, Lee use's your thumb for pushing pressure (gets tiresome) while the RCBS uses your whole hand (like making a fist). If you use a hand primer you can do it just about anywhere , while watching TV or just sitting around a campfire. Using that primer Lee arm means you are tied to the bench and complicates things. To use it you place a case in the holder and pull down on the handle pushing the case into the sizer/de-capper die, the you have to pick up one prime and place in the proper side of the primer tool and then complete the stroke to seat the primer. MAKE SURE YOU DO NOT HAVE ANY OIL ON YOUR FINGERS OR THE PRIMERS WILL GET CONTAMANATED AND FAIL TO FIRE!