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Mexico official overprints can certainly be confusing to distinguish!
What you have is O181a with overprint reading down. Here's O181, same stamp with overprint reading up.
Thank you. I guess I had a misunderstanding of the Scott Catalog. As the group O177-O184 have no color called out for the overprint, I thought the previous major heading called for (Red, O172-O176) carries down. That would be red and mine is black - so I thought, what the heck?!?