OK I was correct. The brass pin holds the tumbler cylinder in the housing and more.
Here's how it works. The tumbler has a pin on the back side on its circumference.
The tumbler also has a slot in the plastic housing to receive the rectangular brass pin I mentioned above. The plastic housing also has 2 slots in it to receive the lock's pins.
When assembled the brass key holds the plastic tumbler housing in place and provides a key in the slot in the housing so it can move in and out without rotating. When unlocked the pin on the end of the tumbler is at the bottom and engages and pushes the latch. When the tumbler is locked the pin is rotated to the top as the tumbler rotates inside its plastic housing and there is now nothing to push the latch.
first pic the pin
second pic the slot
third pic the lock's pins
fourth pic the latch inside
Also refer to pics in earlier posts.
The next puzzle is how to get the unit apart if need be.
Ken Geiger, Toronto
1965 GTA, RHD, Stradale
ex- 1965 GTA, RHD, Corsa, Trans/Am 66-72
1964 Sprint GT, B-Sedan in 70's
1970 Giulia Super (CDN Spec)
1976 1600 GTj (Italian Spec)
1966 Harley Sprint (Aermacchi) 250
2000 Catalina 320, Hull #765 Northern Dream
2001 BMW Z3 3.0i