Parker 21 was first designed as a school pen, a less expensive version of the top selling 51, and it first appeared in 1948. Made of polystyrene plastic, for three years the Mark I, sold pretty well, but had issues with premature ink drying. So, starting in 1952, the Mark IIs appeared, and these became a big seller and model companion to the 51. The Mark IIs had many improvements over the Mark I, gold-filled trim, and a good variety of colors and octanium-tipped nibs. Although the Parker 45 was introduced in 1960, it did not supplant the 21s for five more years. Like 51s, the 21s proved to be very durable, and are easily found today.
The Parker 41 was introduced in 1956, and only survived for two years. It was essentially a brighter-colored 21, and many of their parts are interchangeable. One doesn’t see many 41s these days, and probably didn’t in the late 1950’s, either. Models were changing fast then, and the Super 21 followed the 41 in 1958, and both were superceded by the highly successful 45 in 1960.