why different swapping modes on executables?

[Guy Harris]

>HAHAHAHA! It's even worse than that! The Magic Number was a JUMP
>(actually a branch) across the a.out header. A 407 executable
>had a seven word (14 byte) header, a 410, eight words, 411 nine...
>Sneaky, huh?

*Real* sneaky, given that the header was, for all three types of
executable, 8 words long....

"407" actually skipped all 8 words; the PC to which was added 7*2 was
the PC of the instruction *following* the branch.  "410" was intended to
be "407+1", not "br .+10".