167 buses are interchangeable with 178 perms, thats why
Not interchangeable. When the term "interchangeable" is used, it
normally means something like the old system for Svc
900/901,Svc 920/922 and Svc 941/945/947 whereby the perms
will swop services every single week, maintaining the same group of
buses for that entire week. Example will be something like
this:
Svc 900 and Svc 901 perms (then) were: TIB904A TIB999B TIB1049L
TIB1181L TIB1183G TIB1222C (TIB1245L is out as the bus is perm
900S151 and does not rotate, TIB447C is out because the bus was
never interchanged between both services)
For one week, Svc 900 takes TIB904A TIB1049L TIB1181L TIB1222C
(TIB1245L) and Service 901 takes (TIB447C) TIB999B TIB1183G. This
pattern goes on for a week, then next week the 6 buses that are
interchangeable would form another pattern that goes on for the
next week and so on.
The example of Svc 167 and Svc 178 would not be interchangeable,
but more of a cameo basis because the buses do not rotate around
and stay in the same pattern for a single week. It is just that the
Svc 167 bus appears on Svc 178 for a day, just like how a cameo
should, although it happens more than once. Using the term
"interchangeable" for this example, doesn't it mean that all buses
from all services are "interchangeable" with each other just by
appearing on another service for a day?
As for Merz cameos after UDs came to perm 178, it is not really
counted as rare, but more of uncommon ... ...
