I've recently noticed these round holes on cars before my 1972 which has the storage bins.

What are their purpose? Thanks for any response.

They're only on the series one cars. They had the tools located behind plastic covers just ahead of where the tool boxes are. Presumably someone realized there was some dead space there that could be used to put the tools in and did that instead for the next run of cars. 

The holes are likely a weight saving feature.

Or a way to tease you. You look in there and realize there is space to squirrel things away, but you realize can’t get there from here…. Someone was a hero when they suggested using that space for tools and doing away with the behind seat storage.

The early, early cars didn't have insulation mat melted over the holes in the metal.  I can't think of anything other than weight saving for that design.

As related to me by Uemura san, the pierced dimples 'cheesegrater' deck area is the top half of an important transverse structural (triangular) box section in the bodyshell.

The reason for the pierced dimples is simply to aid strength, and to help avoid 'oil canning' should any load be put on the top surface of the deck. Nothing to do with weight saving. The vertical front face has square-pierced pressings for the same reason (strength), with the earliest examples lacking a partial doubler plate over the transmission tunnel which was soon added to beef up that area.

The subsequent integral tool box design pretty much did the same job, killing two birds with one stone.