Key switch replace?

[BADAZ chevy guy]

1977 C10. My key switch falls out when I pull the key out. I looked at several in the wrecking yard and found they have a screw that goes through the slot on the bottom of the cylinder. My aluminum housing doesn't have a place for that screw.
What holds the key cylinder in place?

[bd]

Early GM ignition tumblers in the column were retained by a spring loaded steel tang that was integral with the tumbler (e.g., LC14290).  The tang engaged a small slot or window in the housing that was about 3/8" long by about 3/32" wide.  Just as a guess - there maybe an issue with the locking tang or someone may have mismatched the tumbler and column.

[BADAZ chevy guy]

Ya, my lock cylinder doesn't have a lock tang.
I think you may be right. I think the PO may have put the wrong cylinder in. My cylinder has a slot that would except a screw keeper. But the aluminum housing doesn't have a place to put the screw.