This is an Esterbrook Plunger-Fill fountain pen c. 1959. Esterbrook introduced the Safari line in 1957, the company’s 100th anniversary, as an all plastic cartridge pen that could hold two cartridges, thus the name Safari Twin-Cartridge Fountain Pen. The new pen continued the use of the 32 different available Esterbrook Renew Point replaceable stainless steel nib units. The cap was changed to a metal one by 1959. Esterbrook took that pen and created a new model with a new name by adding a plunger fill system similar to the one used on the Sheaffer Imperial I. Plunger-Fill pens operate by unscrewing the blind cap at the end of the barrel and pulling out the short plunger. Inside the barrel is a bulb shaped sac that is compressed by air pressure when the plunger is pushed in and the pressure is released at the bottom of the stroke when the end of the plunger reaches a widened space in the barrel, letting the compressed air out through a hole in the barrel and allowing the ink sac to fill. The plunger does not directly compress the ink sac. Instructions for the pen called for two cycles to fill it. The 5 7/16 inch long pen came in six colors: black, navy blue, blue, green, red and gray, and sold for $3.95.
I would love to see photos of the other 5 colors and any ads of this model. I only have the 1959 Saturday Evening Post ad describing this as a new model.