Rulebuster WBC comes out of C. U is a subset of Z so I am not surprised that there is no availability. I honestly think the guy may have been mistaken, though, about Z equally zero.

[KVS Availability Tool 2.8.0/Platinum - WSpan: ITN/US-TAP]
[CODE]MEM Memphis Int'l TN US [KMEM]
AMS Amsterdam-Schiphol NL [EHAM]
FRI 16 Mar 2007

Carrier Flight From Depart To Arrive A/C St Availability
--------- ------ ---- --------- ---- --------- ---- ---- --------------------------------------
NW 58 MEM 19:20 AMS 10:10 +1 332 0 J6 C3 I0 Z1 Y9 B9 M9 H7 Q2 V2 L7 T2 K7