I have an old article by Don Zutz on the Model 90 in which he says the exact opposite of the Blue Book: earlier guns had the midrib but in 1949 the ribs were dropped. Not sure who's right on that score.

PA24, your post confuses me a little. You're not saying that all the Marlin 90's were made for Sears, are you? I've certainly seen Model 90's marked J.C. Higgins (and, I think, Ranger), but I've seen more of them marked just Marlin.