This question applies to two sets 1939 World in Arms by Gum Inc. and the Canadian version 1939 Fighting Forces by O-Pee-Chee.
The set was published with 6 subsets and numbered accordingly "see scans".
The O-Pee-Chee is already listed in the database and is numbered : "Airplanes1", "Airplanes2"....."Ships1", "Ships2"......etc.
http://www.tradingcarddb.com/ViewAllSet.cfm/sid/162277/1939-O-Pee-Chee-Fighting-Forces
I think the O-Pee-Chee versiion is numbered correctly according to the FAQ guidance:
"The card numbers listed on checklists should try to reflect the card numbers as they appear on the cards themselves, including any dashes, prefixes, or suffixes. Unnumbered cards should be entered in as "NNO"."
The only issue I have is the spacing. In the O-Pee-Chee version that is already listed, there is not a space between the subset title and the card #. On the card itself there is a space (see scan). Since we can't put a space in the card# field, should we number the Gum Inc. set like the OPC set is already numbered? Maybe likes this:
Airplanes_1 or Airplanes-1?
My two-cents is worth slightly more than a penny.
-- Chad --