They have not been surrounded by any small splinters of shrapnel so may have been dealt with separately? Could these have been blinds?
Comments appreciated.
They are certainly not the remains of blinds as blinds were strictly disposed of, by either blowing up or burying in a hole 3' deep or more.
Over the years I have had two empty standard Mills No.5 casings (one semi-relic, one non-relic) which have broken in two around the centre line in the process of being refurnished with centre pieces. I recollect the semi-relic did have a slight crack around the centre, and on tightening up the centre piece the casing broke. The non-relic seemed fine, but I remember to my chagrin on tightening the centre piece there was a distinct snapping sound and the body split perfectly in two. I also had at one time a French made (so called Dunkirk) Mills grenade that had a crack around three quarters of the circumference, but I moved it on before the two halves parted on me.
Centre pieces were inserted at the factory with significant force in order to attain as good a mechanical seal as possible. This would have created a permanent tensile force within the grenade body as the top of the striker sleeve pushed on the inside of the crown, and the bottom fuze chamber was set with Pettman cement in the mouth of the body. Flawed casings under constant tensile stress over time might well crack, and the centre line serration groove would be a likely point of failure.
With thousands of grenades being used for training, a few dozen found cracked over time and put aside in one area seems plausible. Levers and pins were retained for use on throwing dummies, so would explain their absence on the discarded bodies. The stripped bodies might have been deliberately hammered open to empty the insensitive ammonal/amatol/alumatol, of which the ammonium nitrate would quickly wash away when it rained.
Several found together in France with pins and levers does suggest the possibility of a flawed batch from one manufacturer.
Occasionally grenades did accidentally leave the filling factory without having been filled with their bursting charge. If not detected before loading with igniter sets, the detonation of the detonator would indeed create a few large, irregular-shaped fragments.