Comment by quuxplusone

(Blog author here.) Nice find! I'll try to incorporate that into the post at some point. The 1951 first edition is also on archive.org (borrowable with a free login account): https://archive.org/details/preparationofpro0000maur/page/32...

I agree, it looks like this 1951 source is using "call in" to mean "invoke" — the actual transfer of control — as opposed to "load" or "link in." Which means this 1951 source agrees with Sarbacher (1959), and is causing me right now to second-guess my interpretation of the MANIAC II (1956) and Fortran II (1958) sources — could it be that they were also using "call in" to mean "invoke" rather than "indicate a dependency on"? Did I exoticize the past too much, by assuming the preposition in "call in" must be doing some work, meaning-wise?