From 1978 on, normal US VONs started with two letters. Canadian and special VONs started with a single letter, eg. E "Export", J "Special Order/Paint", Q "Chrysler Lease/Executive" and B, C or D "Canada". On B-bodies K "Special Order/Police" was still in use. Note that a sunroof car in the list below did not get a special VON.
Two-letter combinations found on C-bodies:
AJ 208
AQ B16
AV 119 Bales Motor Co., Inc.,, Jeffersonville, IN
BC 411
CN 619
CR 626/w M51 "Sunroof - Power"
DJ 327
DR 307
DT 626 (fender tag published some posts earlier)
DU 619
DW 503
EN 517
EP 503
EV 606
EZ 606
ZA 320, 421
ZE 501, 619
As you can see, their occurrence in relation to the Scheduled Production Date is roughly in alphabetical order, but exceptions are no exception, so to speak. A 1978 Cordoba VON starts with FE, which makes me think that combinations AA through EZ were for C-body orders.
The ZA and ZE combinations seem to follow a different logic.
I think the VONs starting with the two-letter combination originate from the dealer's order sheet block. It would be nice to be able to back this up with an order sheet, but I can't.