Oh good grief, folks will make up the damndest conspiracies, all this cloak and dagger stuff. Made up from thin air.
That spot on the back of a military helmet is where the initials of the person for whom the helmet was issued are normally placed.
Y. B. could be the initials for say someone named Yaacov Bar-Lev who is with the IDF. That's a good IDF name.
I've still have my aircraft helmet, the one I bought at a PX, so it's mine, It has my initials on the back like that. I haven't worn it except when climbing and working under trees to trim or cut them down for the last 40 years. It's a darn good snazzy helmet for that kind of work.
They put initials on the backs of helmets because helmet are often stored on hooks on a wall, or are placed in a shelving rack where the initials can be seen so that the correct person can find their helmet amoung all of the other folks helmets.