I dont ever remember that either. I was born in, in 1975, and my memories from, say 1980 and up, of going to the book studies were always wearing what you wore at the "regular" meetings in the KH. This was in America, NYC area, so i dont know if in other parts of the world or country it was different, but i didnt think so.
Lol, i do remember a study bringing her 8 year old son, to the book study, and he was in nothing but a T-shirt and underwear, lol. He had those tighty, whitey underwear briefs too, not boxers. I remember, i was like 9 or 10 myself, and in a shirt, tie, and dress pants, of course, and heres this kid only a year younger than me, in his underwear. I was like wtf, and i remember he wanted to play after the book study. Somehow, i dont think i wanted to play anything with him. I guess i liked my playmates with pants on, go figure.