Paladin's right the firing pin safety is the main difference over the Mark II. I have the Argentine FM which is basically the old Mark I, which doesn't have the ambidextrous thumb safety, 3-dot sights, throated barrel and non-ramped barrel ramp of the Mark II/III. It does have the external extractor though not the weaker internal one of even earlier Hi Powers.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek