I have a Mark 1, vintage 1976. They are the easiest to take apart and reassemble. My son has a Mark III, it and the Mark II have features that the Mark I doesn't. Such as a 10 round magazine vs. 9, and a bolt release/holdback mechanism. The only way to hold a Mark I bolt open is to engage the safety while you hold the bolt open. I can take my Mark I apart and back together 4 times before my son can do same to his Mark III once. You have to follow the manual exactly, hold the muzzle up, remove/insert the magazine, etc., when they tell you. There's a company that makes a kit that simplifies the process: http://www.majesticarms.com/id10.html My son bought one for his Mark III, works very well. I didn't for my Mark I since I didn't have a problem.