K413 Fragmentation Grenade - Grenades are largely divided between the 'fuse' and the 'body' - The 'body' determines the effect of the grenade, while the 'fuse' determines the timer of the grenade.

Delayed Fuse

Striker

Primer

Safety Pin, Safety Clip

Safety Lever

Retardant (Zirconium-Nickel) (~5 seconds)

Detonator Lead Styphnate (LS) Lead(II) azide (LA)

Explosive Filling

Tungsten cube fragments

Enamel painted outer metal shell

Wired fuse (= retardant)

Izuna

Explosive body

Tsukuyo

Steel wires

As shown above, a fuse that utilizes a slow burning stack of gunpowder is a "flame retardant fuse"

The fuse bomb used by the students of Hyakkiyako is most likely the predecessor of the delayed fuse grenade.

Furthermore... The safety pin is rather difficult to pull out, so I do not recommend that you bite it and try to pull it out with your teeth, Sensei.