1: Chimney Holes: Zippos from '34-'46 had 14 holes (7 per side). 1947-present have 16 holes (8 per side).
2: Striker Wheel Mount: The mounting for the striker wheel was like a half-circle shape on '34-'46 models, and then like a half-teardrop shape from '47-current.
3: Hinge Barrels: '32-'35 had 3-barrel external hinges. '36-'37 had 4-barrel external hinges. '38-'44 had 4-barrel internal hinges. '46-'49 had 3-barrel internal hinges. And '50-present have 5-barrel internal hinges, except for some special editions I have seen with 4-barrels, but are newer replicas with the appropriate date codes on the bottom. A 3 or 4-barrel lighter may have been sent in for repairs, and fitted with a 5-barrel hinge, making the date harder to determine.
4: Flat or Indented Bottom: '46-current Zippos have an indented bottom unless it's a replica. Up until '45 the bottoms were flat or curved outwards. There are some exceptions to this (especially slims), but any flat bottoms after about 1955 will have a date code.
5: Bottom Date Codes: It would take a while to type all the date codes here, so here's a quick rundown: '32-'35 have PAT. PENDING in the lower left corner. '36-'41 have PAT. 2032695 lower left. '42 had an error and PAT. 203695 lower left. '42-'55 have PAT. 2032695 lower left, with some '53s having PAT. 2517191 and PAT. PEND together. '55-'56 have PAT. 2517191 lower left with 3 or 4 dots on each side, too. '57-'65 have PAT. 2517191 on center bottom with the number of dots reduced each year by one. '66-'73 date codes are vertical lines "||||" reducing by one each year. '74-'81 had forward slashes "/ / / /" reducing by one yearly. '82-'86 had back slashes "\ \ \ \" reducing by ones. '86-2000 used month letters (A thru L) and year Roman Numerals: II = '86 and XVI = 2000. 2001-current have month letters and regular numbers: L 06 = December 2006.
I hope this helps you determine the approximate age of a Zippo before you buy it. The quickest hints are barrels, chimney holes, and indented bottoms. Good Luck! Please Click "YES" below if you found this helpful! Thanks! Don C.