Agreed @tenkan, but I always associated 13 batteries, as P13, I have no idea what the “P” stands for though? I have distributed thousands of these whilst working with Deaf Services, alongside 675’s, 312’s & 10’s, but the “P” intrigues me, I will Google it… 
Edit; The P is type of Designation from PowerOne manufacturer they use for P10, P13, P312 and P675. Gospel according to Google… 