@ Jay ...
All of my thoughts exactly ... the P keyway illustrated in the keyway-
hierarchy graphic is the same as what Ilco, GMS, et al, refer to as
the "Schlage C-K keyway," and it's exactly the same thing: a
composite cylinder milled specifically to accept all Classic Obverse
sections, C through L. I can imagine that such composite keyways can
make things infinitely easier for lazy locksmiths, facilities
personnel, and such, as the composite keyway could relieve the need to
stock the 7 Schlage Classic Observe keyways, but ... yeah. Septupling
the number of keys that could potentially (unintendedly) operate a
composite cylinder alone is a very good reason to stay the hell away
from this option. In masterkeyed systems, even in very large
multiplex ones, I can't think of a single reasonable argument for
using the composite keyway ...
If, say, the cylinders used are in the E, F, and G keyways, and change
keys are cut onto the corresponding blanks, mid-level masters cut onto
H, J, or K blanks, and high-level masters on the L blank, it'd
certainly be easier to use the P keyway for a cylinder to be
extensively cross-keyed across sections, but it seems to me that
common sense and ordinary security policy would dictate using another
keyway entirely (say, C if key duplication isn't a big concern, and
maybe FG if Walmart key duplication was to be pre-empted), key it as a
change key under a/the GM or the GGM, and issue a single key for the
operation of that cylinder.
In large facilities and institutions, on campuses and in large medical
centers, for example, which often do use multiplexing, I've most often
seen perimeter doors for a particular building keyed alike, as a
change key directly under a GM or the GGM (i.e., A1, GGM1). On
college campuses, classrooms within an academic department or within a
given building are very often all keyed alike. Still, in any of these
cases, I have some difficulty imagining how the use of a composite
keyway could be justified from a security viewpoint.