The ruling elders were Congregation Servant [the boss], Assistant Congregation Servant [filled role similar to secretary], and Bible Study Servant [Promoted bible study activity]. They were the congregation committee supervising congregation functioning and 'judicial' matters.
The assistant and bible study servant were called ministerial servants. Other ministerial servants were Literature Servant, magazine servant, accounts servant, book study conductors, watchtower conductor ministry school servant. In most congregations these positions were filled by people already appointed to another office. The Congregation Servant might also have the watchtower study. The assistant might also have the ministry school. Book study conductors usually had another office. This is, i believe, how things stood in 1971.
Earlier eras had differing designations, though they performed the same work.
The watchtower book Your Word is a Lamp to my Feet would help with this. It shouldn't be hard to find.
Old Goat who sometimes posts here [he's 90 I think], was a Company Servant, the predecessor title for Congregation Servant. You might PM him if you need more information. But the 'service' booklets and books are mostly found online somewhere. Other than the Lamp book, these would be Preaching and Teaching, and the predecessor Organizational Instructions.