As far as I know, the leather box was used for the first Curvex models beginning in late 1935. There is a round paper insert that was fitted into the right side cover which described "introducing the Curvex". Kyle Rogers has a complete set of men's and ladies with the paperwork.

The round blue plastic box was used for later (1937) models, including the Majesty and (with a different holder) the 330 Rist-sides. There were other versions of the blue plastic boxes used until around 1940, and the round plastic box makes a return (in gray and maroon) in the late 1940s for the 440 Rist-side models.

But, since we're talking about Gruen, it would not surprise me at all if there were early 330 models sold in the leather boxes and late 311 models sold in the blue plastic boxes.