In message 01cd1ce7$0b1bb240$LocalHost@default, at 22:33:35 on Tue, 17
Apr 2012, Michael R N Dolbear remarked:
no one on uktl from that day to this has queried that all possible
caps can apply all the time so a Z1-4 cap is quite independent of any
Z5 or Z6 journeys that may have been taken.

Until the OP's example, of course.
--
Roland Perry