In Yugoslavian People's army there were four different types of every type of antipersonnel and antitank mine:
1. Live (with explosive charge and initial part of the fuze)
2. Drill (with changable smoke charge and drill initial part of the fuze (incendiary cap for starting smoke charge)
3. School (Classroom model with inert explosive charge and inert initial part of the fuze)
4. School cutaway model (classroom model sectioned that 1/4 of the body is missing and it shows all inside parts of the mine and the fuze)
Markings:
1. For live mines markings are made with yellow or sometimes black letters (for example PROM-1)
2. Drill mines were painted as live in olive-grey standard YPA paint, with yellow band added on the body or on the fuze. Markings were the same, but with added prefix letter "V" what means drill, or "vežbovni" (for example VPROM-1)
3. School mines (classroom models) were painted the same as live, without yellow band, but they had added hole or holes on the body of the mine and on the fuze. Markings were the same as on live, but with added prefix "Š" what means "školska" or classroom model in english.
4. Without any markings or rarely markings the same as on 3.
you have school model of the fuze UPM-2A and UPM-2AS, prefix "š" means "školski" or classroom model
I hope that I helped

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