Weird sheet align marker - a circle? on consec 1935A $1 SC Mules

I am used to seeing cross hatch and line style sheet alignment markers but not circular.
Per recent SPMC booth conversation, nobody had seen these either, so lets open it up to the group, picked this pair up 2 decades ago each with a half circle on the right edge. Any idea what it is?

Has anyone seen a full or half circle used on any sheet alignment marker. If it isn't a marker, what is it?

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