Mint or used.
All I found after a quick check was just one topic with an image:
https://goscf.com/t/25154Mine:

Mexico City to Puebla, 1895.

R.F. RIVEROLL/MENSAGERO N oval, 1898, to Mexico City. I'm assuming this was either part of a pasteup where several envelopes are overlapped for multiple weight rates (though usually found cancelled only but unaddressed, or was just attached to a letter that carried the full address. The end of the flap is missing and was likely cut off whatever it was attached to.
Both are blank on the back, which seems to be the norm for these.