Such detail omissions were pretty common when those kits were tooled--consider that the tooling process for say, the AMT '40 Ford Coupe kit probably began sometime in 1958 for its summer 1959 release--and most likely the draftsman who did the drawings missed those hinges (did they even show up in such reference pics as were taken prior to the start of the drawings?). In addition, regardless of what we might think today, pristine examples of any 1940 car would have been a bit more difficult to find in 1958-59, due to their being just "old cars", most that were running (Fords in particular) likely were teenager rides--restorations to original really weren't around of cars that were 18-20 yrs old.
As for pulling from the mold(s), bear in mind, if you really look at a '40 Ford coupe (or sedan for that matter) body, you will readily see that it was molded in the same sort of 6-way sliding-core molds as any one piece bodied contemporary (1958-59) AMT 3in1 car kit. but using just 5 mold slides--front, inner surfaces, right side, left side, and upper surfaces. That made all the mold sections "pull away" from the molded body shell rather than the entire body being somehow "popped" out of a single cavity.
Frankly, I would submit that rather than say, working from factory photo's and drawings as would have been the case back then with kits of 1959 production cars, they more than likely worked with basic dimensions, and photographs from say, Ford Archives and such magazine photo's as were available--quite unlike the extensive and intensive research that goes into model kits of out-of-production cars as is done today.