Early 4 Screw guns are numbered just like 5 screw guns in most models:
Butt, barrel, cylinder, yoke, and extractor.
Not far into the 4 screw area when hard fitting became universal, the barrel, cylinder, and yoke numbers became unnecessary and you don't pay people to do things you don't need to do.
The extractor number continues well into the 3 Screw guns.

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Regards,
Lee Jarrett