Unusual marks on an old photo

Recently I was asked to restore a photograph that had some unusual fine white pin marks on each corner.   I have not seen these types of marks before....

The full restored photo

Enlarged pin mark

The marks remind me of crop marks used by print designers when laying out publications either to indicate page edges or where to crop an image. I am wondering if these marks were added by the photographer to assist the picture framer. So far my research has not been able to confirm this. Have you seen this on any of your old photos do you know what these marks indicate?

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