Try and find the article explaining why they changed from "servant" to "overseer."
The phrase "congregation servant" disappears by 1972 with the institution of the elder arrangement. The term "congregation overseer" does occur prior to 1972 in print but was not used verbally until 1971 to 1972.
While the WTS documents that "servants to the brethren" changed to "circuit servants" in 1948, there is no similar calendar landmark for the change from circuit servant to circuit overseer.
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dx86-03 Dates (Calendar) ***1894, pilgrims, jv 223
1928, pilgrims renamed regional service directors: jv 223
1948, servants to the brethren renamed circuit servants: jv 223