Hi Barb:
Somewhat after the barn door was left open and the horse got out . . .
Apparently the baptismal candidates were not sufficiently versed in Bible/WT teachings prior to 1975. Brother Knorr announced that the "Lamp" book and its coverage of JW articles of faith (my words) and the follow-up questions would help stem the ongoing problem: 'People are coming into the Truth through the front door and then leaving through the back door.'
Just a side point I recalled after seeing your question. Were you at Bethel prior to that book's publication? I was there circa 1970.
Best.
CoCo