Somehow the Mutual Book Company of Boston, a somewhat cheapsie sort of outfit I gather, got the rights to an Opper postcard image in 1905. Also rather odd that Opper, if he was in fact the author of the caption, chose to appropriate a very similar term to one popularized by his Hearst compadre, F.M. Howarth (Mr. E.Z. Mark). But I doubt Opper chose the caption, because I was able to find one other Opper postcard in this series, and that one appropriates the gag from a popular postcard series of the time “The Whole Dam Family”. Seems like the folks at the Mutual Book Company were trying to play piggy-back on proven sellers.
The cartoon itself is a bit opaque to me — I guess Opper might be saying that oysters served at a church fair are a good bet to make you sick (you’re a Mr. Easy Mark). If that’s the gag, it seems like the oyster might have been drawn to make it obvious it is noxious. This one looks like a fine upstanding citizen.