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.
Ross