221 Ideas Book No. 1 from 1973. In my experience their investigations take a couple of weeks.
Vintage Lego brochure - Ideas Books 221 & 222 (1975)
Vintage 1973 LEGO Instructions Ideas Booklet 221 Number 1
LEGO 1973 Ideas Instructions Booklet Vintage 221
LEGO Technic Ideas Book 8889 Building Project Ideas
Lego 1970s vintage manuals and booklets. Lego 200, Lego 221, Guide to Lego 1974