The M17 "dashes"
-1: Right-hand to left-hand threading for extractor rod in 1959
-2: Delete triggerguard screw (three-screw introduction) in 1961
-3: Relocation of rear sight leaf screw forward of forcing cone in 1967
-4: Change gas ring from yoke to cylinder in 1977
1982: delete pinned barrel
1986: introduce four-inch heavy barrel for M17; delete M18
-5: New yoke retention system in 1988
-6: Add full-lug barrel in 1990
-7: Drill and tap frame for optics; change rear sight to round base; change extractor in 1994
