This is a comment on babou's answer, in response to his suggestion to check the magazine serial. I am posting it as an answer because it's too long for a comment. Meanwhile, in the unlikely event that babou finds anything herein that would be a useful addition to his answer, he is welcome to copy it. Personally, I don't see the relevance of the magazine serial to the OP's question. I don't know how the magazine version is related to either the published book or the manuscript submitted to Scribner's, but my uneducated guess is that the magazine version was created by abridging one of the full-length versions to fit the space limitations of the magazine.
The answer is that "thirty-one capital offenses" and "thirty-one seconds" are in the magazine serial, "Form Thirty-One" is not. Form Thirty-One is mentioned in section 13 of the book Starship Troopers; sections 12 & 13 of the book are condensed into one section (section XI) in the serial, and the whole part where Form Thirty-One comes up is missing.
This is from "Starship Soldier" (first of two parts) in The Magazine of Fantasy and Science Fiction, October 1959The Magazine of Fantasy and Science Fiction, October 1959 (available at the Internet Archive), p. 141, column 1:
Before church call every Sunday they lined us up and read aloud the disciplinary articles of the Laws and Regulations of the Military Forces. They were posted, too, outside the orderly tent. Nobody paid much mind. About the only thing we noticed was what we called "the thirty-one ways to crash land," the thirty-one capital offenses. Now and then somebody boasted of having found a thirty-second way—something preposterous and usually obscene.
And this is from the second part, in The Magazine of Fantasy and Science Fiction, November 1959The Magazine of Fantasy and Science Fiction, November 1959 (available at the Internet Archive), p. 94, column 2:
"Thirty-one seconds, Lieutenant" She added, "Good luck, boys! This time we're going to take 'em!"