World Stamp Catalogue/United States/Catalogue numbers
Tentative numbering system for US stamps 
Based on World Stamp Catalogue/Numbering System the following numbering system is being tried for US stamps
Format: CCC - YYYY - SSDD - TTKK --- ---- ---- ---- Example: USA - 1851 - 0101 - 3201 --- ---- ---- ---- where: CCC = 3-letter country code (USA = United States) YYYY = 4-digit year of issue (1851 = Year of issue) SS = 2-digit series order (varies by year) (01 = first (and only) series for 1851) DD = 2-digit denomination order (varies by series) (01 = 1-cent, 02 = 3-cent, 03 = 5-cent, etc.) TT = 2-digit type and subtype (varies by denomination), (31 = Type III, 32 = Type IIIa) KK = 2-digit color variation (varies by denomination), (01 = first (and only) color for this denomination (blue))
This numbering system will likely change to some extent as more stamps are catalogued. This system will probably not apply to local, carrier and provisional stamps issued in the US.Ha!