Hi Francesco,
I do completly confirm this markings on Polish grenades and want to add:
1. F1-classroom-trainers have always a white horizntal ring around the center and sometimes an additional vertical white stripe on front- and rear-side.
2. Bottom of classroom-trainers has no hole.
3. F1-practice with training-charge for sound and flash, Ive seen black bodies with red horizontal ring, olive-drab bodies with red horizontal ring, but also olive-drab bodies without other paint-marks, all three version do incooperate a hole in the grenade-bottom
4. Polish F1-grenade-bodies could be sometimes (first years after WW2) Russian-production with Polish-made fuze, but mostly Polish-made grenade-bodies from "early-type" which does have fragmentation-grooves also in the grenade-"neck" and "-bottom". This early type has on one side on the upper fragmentation-row a cast-in mark which is an standing oval ring with a number inside and at the opposite side at the corresponding spot the letters "PFA" (many times filled with paint, and hard to find).
The newer Polish-made F1-bodies do not have a serrated "neck" and do carry the factory-cast-in mark on one side around the center, aslo an oval ring with the number 31 inside, but here the ring is hrizontal (laying) and no other factory-marks.
5. Note that the thread of fuzes for classroom-trainers and practice-versions are slightly different in size
Note: all issues had been seen on 100% INERT (FFE) items only, which had been already in this INERT condition.