It's not an
ebay listing #, but it might be that seller's personal reference number. A number of sellers will have their own numbering system for their stock. I couldn't find any reference to WPX.
As for the APS, someone else will probably know more than me. I don't know how much they have online. (edit - I just checked their website - they have the catalogs, but not readable online).
This is some basic information about the catalog you can use for general information about worldwide postal stationery:
http://en.wikipedia.org/wiki/Higgin...nery_CatalogFor many countries there's more specific catalogs on a country to country basis, i.e. if you're into German postal stationery get the specific Michel catalogue and so forth. It can get expensive to try and get them all. Hopefully, the APS can help out - or-
Again,
ebay has a seller selling the whole thing digitially for $100
http://www.ebay.com/itm/Higgins-Gag...251212486701 - well over 2,000 pages, but hasn't been updated since the 1980's.
You can also buy it by the country/region as needed.