I have a couple of old dummy grenades I picked up at an army surplus store sometime back in the late 80's when I was a kid. As far as I can tell these were real grenades at one point that had been demilled and sold off. They are the pineapple style, what I believe was called the M26. I'm not sure but I believe based on the colors of the spoon that they were likely training grenades. Most of the spoon is painted a light blue with brown around the bottom. The fuse (or whatever the portion is called that screws into the top is OD green.
Anyway, one one side of the grenade there is the number "5". Does this denote it had a 5 second fuse? Then on the other side are the letters "RPX". What does that stand for? Any other information about the history of these grenades is also welcome, as I've long been curious about them. Other than the big hole that was drilled through the bottom of them and the inner goodies removed, they look like the real deal.
The sad thing is, I remember a time when I could hang one of these on my belt and take it to school with me and nobody cared. I even remember doing a 4-H demonstration project with these, an old steel pot helmet, a demilled .50 caliber round and some other stuff. If I were a 10 year old kid today did any of that, I'd likely find a guy in BDU's standing on my neck, a gun in my face and my time in public schools over. My, how times have changed.
They are training grenades, but aren't "demilled" in the sense that they were real at one time. They were made from the begining as a training device, usually on the same production line as real ones. Training devices frequently use many of the same components as real devices in order to simulate weight, and operation. The training Claymore mine actually has 700 steel balls fixed in the same matrix as the real one. It just has an inert filler. It sounds like more work to make, but really on a production line situation, it ends up easier and less expensive.
The MK2 was the "pineaple" of WWII. It used a cast body with a TNT filler. The body caused the fragments, and the grooves that give it the pineapple appearence were placed to allow for a more uniform break-up.
The M26, which has a smooth side, actually has a wound prefragmented insert that goes inside the body and the filler is in it. The body of the grenade itself isn't the primary source of fragments, but the "wire" insert is. This produced a much larger number of fragments, though they were smaller. The end result was a more effective capability to produce casualties within the blast radius.
The M33, which is more round, is designed the same way. More modern fillers, and it's better shape make it easier to throw, and no less effective.
The fuze is "about" 5 seconds. They aren't exactly the most precise things around, so you can't "cook it off" to 4.75 seconds and expect it not to go off in your hand. The timing relies upon chemical reaction and not a quartz timer.
Most handgrenades also have an adaptor developed for them, to allow the user to shoot it off his rifle. A rifle grenade adapter on the end of an M26 can be fired off the flash hider on the M16 series. The US pretty much doesn't use these anymore, as the M203 40mm is much more effective, accurate and goes about 4 times farther.
Some of the same type grenades were fitted with impact fuzes. As long as you threw the grenade over a specific height, the impact would set off the explosive, so the enemy couldn't throw it back. They had a back-up delay incase the impact wasn't enough, or it landed in soft mud, etc. These can't be fired with an adaptor, as the jarring from firing could set off the fuze.
There are also other types of grenades, like smoke, riot agent, WP, Illum, Thermite, concusion, etc.