Possible Clues:

• Nine-digit IDs, for short
• 1040 figs
• Double-hyphenated IDs
• Info collected by H.R
• Figs. assigned randomly since 2011
• Key figures at the IRS?
• ID nos. that shouldn't be shared widely
• Figs. at the beginnings of tax forms
• Info on tax forms
• Nine-digit government IDs: Abbr