It turns out that one can purchase the drive pin from Volvo - PN 1389935.

Volvo also sells a (relatively)low-cost distributor repair kit which appears to contain some washers including the toothed cover plate and at least one shim.
One cannot tell much from the Volvo parts pictures. I ordered one and will let you know what parts it contains and how it works. Since it is nearly impossible to satisfactorily flatten the toothed seal cover washer after prying it out, a new one would be a great help in maintaining a smooth, flat bearing surface for the outside phenolic washer to prevent accelerated wear after re-assembly.

The kit is PN 3501194 and is under $8 from Tasca Volvo. The pin was $1.22 which is a lot less than I spent to buy a whole box of 5/32" x 1" dowel pins (screwy size not generally available from even good hardware stores) that I still had to cut down to use. Based on the difficulty in removing the pin without deforming it enough to cause problems on reassembly, I would certainly recommend having a new one (or two) in hand before you re-seal your distributor.