Who is supposed to define MASTER_SITE_SUSE91 ?

I see it is (now) used in all the emulators/suse91_* pkgsrc
Makefiles, but I can't find a definition for it anywhere (mk/bsd.sites.mk
only has MASTER_SITE_SUSE).

Did a checkin for this one get forgotten somewhere, or is this
something that individual sites are supposed to work out and
set in /etc/mk.conf for themselves (and if it is that, why
is it not mentioned in the pkgsrc.txt or any related files?)

kre