NetBSD-5 appears to have forgotten how to execute 0.9A binaries

[buhrow%lothlorien.nfbcal.org@localhost (Brian Buhrow)]

        hello.  I have a server which was, up until yesterday, running
NetBSD-4.0-stable.  After I updated it to NetBSD-5.1 with sources from July
18 2012, I find my binaries from NetBSD-0.9A no longer run.  The problem
seems to be that the file loader doesn't flag them as being a.out
executables, and the a.out emulation engine isn't turned on.  I have some
other binaries from NetBSD-1.2, which work fine, so it's not entirely
broken.
        I'm not asking for this to be fixed in the main-line tree, but can
someone give me a hint as to where the code is that decides which emulation
engine to start in the kernel when an executable is run?  I figure I can
probably fix it in my tree, file a bug, and if anyone else needs it, they
can patch their tree.
-thanks
-Brian