Holes in roof for teardrop marker lights

[VA72C10]

I'm sanding my truck (72 Cheyenne Super BB) down to bare metal to prep for body work and paint. In the process of sanding the roof down I found a hole...then another....and then a pattern emerged. 5 sets of 3 holes. One further back, one larger and then a smaller one a little forward of that. The roof is flat and nowhere on the spid does it show clearance/marker lights for the roof. So it's not that (plus they wouldn't have had three holes)

Any clue?

It was under what I assume was the factory primer but was filled in with filler....didn't think they used filler then....

Any info would be appreciated.