Both my 81 ProRacer and my 84 ProRacer-S
have those holes.
Showing the 81 ProRacer:
Topic on this bike, with many pics
My ex pro 80 Koga Miyata FullPro-L even has 'M's cut out the shell, like shown on this bike in the 1978 Miyata Jp catalogue:
Here, in the Retrobike Gallery
As Stick Legs says, it probably has to do with weight saving. Moreover the Japanese are very into perfectioning and detailing and that will pretty sure also have something to do with it.
More Japanese brands do this, for example Nagasawa:
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