I can fully vouch for TMS's description of the "arrangement" prior to 1971.
In a congregation the only "elder" (presbyter) was the Congregation Servant. The Assistant Congregation Servant was termed an "assistant ministerial servant" (diakanos) - as was the Bible Study Servant. These were the service committee.
Also the Watchtower Overseer and School Overseer were "assistant ministerial servants" but unless there was illness or someone being away for a long time or it was a close relative being dealt with then the three brothers described by TMS always did every judicial committee.
You need a hard copy of Qualified to be Ministers (1967 was the last copy I think) to check I have this right.
I was just a kid but even in our hall at the time where we had several baptised men our Bible Study Servant (forerunner of service overseer) in 1970 was a keen pioneer of only about 20 and so would have been on the judicial committee.