The "true" attribute names in the 5e character sheet are something of a mystery. I typically find multiple candidates for ability check bonuses and saving throws. The attribute names are also different between PC and NPC/Monster character sheets. As far as I can tell from reviewing a Monster character sheet the attribute name for an NPC's proficiency bonus is "pb", although it might be "npc_skills_flag" (these are theoretically listed on the "Attributes and Abilities" tab of the character sheet, although I have found that this list is not always exhaustive).
