The Diplomat was introduced in the Spring-Summer 1941 Catalog and continued into the 1942 season. I guess you would call it a "mid-season" introduction.
The Diplomat is a 6-tube bakelite set with a fancy cabinet. The dial on these radios is red, providing an interesting contrast with the white cabinet. Many people are now repainting these radios in a variety of colors, and sometimes painting the grille in a second color for contrast. To my knowledge, the Diplomat only came in white. It was not even offered in plain brown bakelite. The Spring-Summer 1941 catalog picture lacks a Firestone Air Chief decal on the front of the radio. The Fall-Winter 1941-42 catalog shows the radio with a decal. There may have been two versions. A radio without a decal may (or may not) have been repainted.