Would not suggest adding one if your car only has two. Read reply #8 below
Took a quick look only through my 68 Dearborn cars for the holes and inserts and found only two inserts (one unfilled hole) on all of the visible examples. Did have one that is difficult to make out and it may have three.
To show the span of this quick look partial Vin's are posted below. Of course with most cars the area is not visible - engine, master cylinder and the like get in the way. All the following unrestored, apart, in a yard, basically not messed with it appears
8F01C1245xx
8F01C15723xx
8F02R1718xx (pictured above)
8F02R1943xx
8F02R1982xx
8F01C2107xx
8F02R2068xx
8F03C1985xx - may have had three
Started to go through San Jose and the pattern (only two) was how it started out. Will get back to it but changed focus since the OP has a Dearborn R code