UO Ortho- unusual marking

Hello eyepiece experts, I just added the 9MM UO Ortho(non HD) to my set of 5,7,12.5,18 and 25MM's. The 9MM is the only one that has additional lettering along the top. The unusual letters are
"O.P.S.". Can someone please clarify these letters? Thanks.

Kurt,

OPS refers to Orthoscopic Planetary Series. I don't know much about them, but they are supposed to have some kind of filter coatings on them to enhance planetary viewing.