Esterbrook manufactured a "school" pen similar to Sheaffer and Wearever in that they had cheap soft plastic barrels and thinly chromed caps. Esterbrook made a number of configurations other than the standard cartridge pen however, including inexpensive screw caps, lever fill pens with friction fit or screw caps. These pens had black plastic sections, unlike the M2's which had color matching sections. The clear pen at top was a standard issue model and not a demonstrator pen like some ebay sellers might lead you to believe. In my experience, green is the hardest color to find in the series. Note, I do not know of the official model designations for models with screw caps or with levers, CX-100 is the cartridge fill friction fit cap only, but for lack of better information, I classify the others as members of the CX-100 "Family"
Of course, companion mechanical pencils and ballpoint pens were available. The pencils were similar to the later safari pencils in that they utilized a twist fill mechanism rather than the cap activated model Esterbrook was most well known for. Pencils and ballpoints for these models are very hard to find.