Given that it's a repainted piece of military hardware, back then that lettering would likely have been rub on transfer. Sometimes it was custom, but WAY more often it came on sheets. many folks here know this old tech, but it's worth repeating for those who don't.
View attachment 1561971 There were always multiples of the popular letters, less of others. Running out of letters was a hazard. Having used these on many props early in my building life, I had this problem many times. I can EASILY imagine a circumstance in which some PA was given this low-skill entry level job and they ran out of some of the letters they needed. And them ending up abbreviating here and there for efficiency's sake. In the melee of a film set that kind of thing happens and slips through the cracks all the time. That would handily explain the correct spelling on one grenade but not the other.