Thanks for the great question. I'll answer it here and on the Q&A page of the web site. The "Pen Gun" flares you were issued in the early 1960s were made by Penguin Associates, Inc., Malvern, PA and issued as MK 79, Mod O Signal Kits, Illumination, in ziplock plastic bags. I carried these during my deployments to Vietnam, as did all Navy flight crews. The individual flares did in fact screw into the pen gun launcher - named that because it resembled a pen for writing - and the empty fired case had to be unscrewed and discarded before a fresh flare could be loaded. Or, if the flares were all taped together, the launcher was unscrewed and moved over to the next live flare. These flares were loud and had a lot of recoil, about like a .38 special, and went about 400 feet above the ground. The flare composition - red only - was fired out of the flare case, similar to a cartridge bullet. The flares would not penetrate a jungle canopy, which is why MBA designed the M201 "foliage-penetrating" flares, first in red and later in white and green. The U.S. Air Force and Army bought millions of them, and they made a lot of profit for MBA, the only Gyrojet to do so.
DK's MBA launcher is a Model 207, the last of the series which is still in use today. The 13mm MBA flares were nearly silent, had almost no recoil, easily penetrated the jungle canopy, and went up to about 1,000 feet, depending on how much foilage they had to penetrate first. The MBA flares did not screw into the launcher's mouth. They were held by friction, and when they fired, the whole rocket launched with nothing left behind.
Chapter 15 in the book, 18 pages, covers all the flares, including smoke, chaff, and infrared and all the launcher variations with color illustrations of everything.
There's an interesting story, not politically correct, I did not include in the book. In the early and mid 1960s when I was in and out of Vietnam, U.S. military personnel in civilian clothing in Saigon were not allowed to carry firearms. Some of them decided to carry a pen gun, cocked, with a flare loaded for personal protection and a few extra flares. In one case late at night, after he left a bar, a U.S. soldier in civies was chased into an alley by three locals, Viet Cong or not, possibly just to rob him, but who knows. As the three approached him, the soldier pulled out his survival flare gun and fired a round at the first attacker. The burning flare composition stuck like glue to the bad guy's chest as it burned a hole right into him. His screams of pain convinced the other two to "beat feet" as they say, and the soldier's life was saved without the use of a "firearm."
The Navy didn't buy the MBA flares because they did not begin their pyrotechnic display until they were several hundred feet high. The Navy had to have something that started burning right at the surface, so a man overboard or a pilot in a life raft's position could be pinpointed at night. There is little jungle foliage that needs to be penetrated over the Gulf of Tonkin or the South China Sea.