It's most likely from old and worn magazine light seals, where you insert dark slide on the magazine.
It will be great if you can include in your picture entire negative with margins?
Generally I'm trying never to leave dark slide in the magazine while camera is in storage mode. Dark slide left in magazine ruins light seals much faster. YMMV
With new film inserted, put black electrical of gaffer tape over the dark slide slot and try it again in bright sun.