Magazine Lock : Early on, the magazine went through slight design changes and it appears at about the time the type 4 magazines were introduced (somewhere between s/n 14,000 to s/n 20,000 being a realistic number), the magazine lock was also changed. It is not sure if this was a full blown recall or not, but in any case any gun sent back to the factory for magazine problems, it was returned with a new magazine lock installed and a replacement magazine.
This new lock was about .015 longer, to match the newer magazines. Therefore a early magazine will not usually lock up in a newer firearm. And of course if you try to use a new style magazine in a old gun it will be looser on the front and won't feed ammo properly.