The M16A1 rifle used the solid rubber butt plate into the 1970's. I used rifles mareked XM16E1 and M-16A1 with such a butt and resisted changing the butt on my then assigned rifle to the trap door design in Europe in the mid 1970's. The frap doors started coming in as a replacement part in about 1974 and chipped and cracked Rubber Butt Plate stocks got replaced as needed with the "new Stock". The early trap doors seemed much more fragile when dropped butt first on concrete, especially in the cold where they would crack around the heel and into the area of the screw hole that held them on to the recoil spring tube.
The trap was supposed to house a sort of triangular cotton pouch with a cleaning kit in it, but did not come with the kit as a replacement part. Troopers found a partial pack of smokes (about 75 percent of the guys were smokers) and a book of matches fit in there with room to spare, Other notice there was room for various types of pogie bait.
The cleaning kit before the butt trap stocks was a rectangular nylon pouch with an LBE ALICE clip on the back that rode on ones pistol belt or stuffed in one's butt pack. Some contained a medium bottle of LSA (Lubricant, Small Arms) a petroleum based lube I blame for most of the M16A1 failures in Europe. But there was ample room for all the rod sections, the handle rod, the patch eye, at least one bore brush and one chamber brush (lucky folks still had the paper tubes they came in as brass bristles could puncture the case and you shirt and under shirt and emidermis) and the Green tooth brush (which I once actually saw some one use as a tooth brush, uugh) I also stuffed into my a Lube and grease tube from an M-14 cleaning kit with the small end full of grease (the M-1 and M-14 lliked a bit of heavy grease in a couple of spots) and the big end full of PL Special (the Petrolium Lubricant, Special used for our M-60 MGs and which worked MUCH better below freezing than LSA on the -16s) There was even room for a piece of wash rag and some patches (7.62 patches we precut into quarters) and a uncut patch wrapped around one each of firing pin retaining pin, extractor retaining pin and extractor spring the most lost parts ( and yes they came in handy when a member or three of my squad lost one.
Our E6 Squad leader insisted we carry as a piece of squad equipment and ammo can (on marches that thing seemed all edges and seemed to be full of Tungston or Uraniium) that had two rifle cleaning kits, an M-60 Cleaning kit, M203 Brush and thong, a demo kit knife, Two spare 20 round magazines and a spare 1911 mag and as many and a variety of M-16A1 parts as we could acquire, the retaining pin for the cap on the back of an M-60 bolt group, a trigger group pin and the retaining spring and the Firingpin, its spring and the "hammer" (a steel tube that went in the bolt of the M-60 to position the firing pin and spring properly) for an d M-60. A towel cushioned the stuff to prevent ratting. He also insisted we carry one of the large ammo cans 5.56 ammo came in full of water. Weird guy that one. Boy that second can WAS heavy.
A few years later after I was commissioned I put together a duplicate of that squad can that went to the field in my jeep trailer.....actually caught flak for it,believe it or not.
-kBob