I was aware the book listed 1020 for the bbls on that page. Where it is stated as 40 points of carbon is on the page for the bbls only. Personally I feel that when this chart was put in it was simply misstated there as being 1020. 1020 would be a viable option for most of those other parts, most of which were going to be case-hardened, but would not make a particularly good bbl.
4140 does date back a long ways & well could have been used. It is also quite possible different alloys were used over the years or even simultaneously on different grades, but they would all likely have contained that approx 40 points of carbon. The points was incidently an old method of stating the % of an alloying component, 100 points equaling 1%.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra