I tried to post the entire Alldata page which discusses various repairs and changes to the door locks with no success. I couldn't get the image on the page to copy into this message. However, here is a copy of the statements from Alldata regarding repairs when the cylinder caps are missing:

A. Lock Cylinder Cover Plate Loose or Missing
If the lock cylinder cap (arrow) is loose or missing, the entire lock cylinder mechanism must be replaced (Figure 1).
Note: The lock cylinder mechanism number must be included when ordering parts.
As of February 1987 (production phase-in) the lock cylinder caps of the front doors and the trunk lid are swaged (previously peened over)

Have you found a source for just the caps?

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog