The serial numbers on Orange amps have known a few different numbering systems.
Matamp Oranges

Orange Matamp OR100?? 005 Sept 1969
Orange Matamp OR100?? 119 Jan 1970

Orange Graphic - pics only

    GRO50 pix only GRO50 151
    GRO100 pix only GRO100 449
    GRO100 pix only GRO100 568 (cab OA3010)
    ORS100 pix only    
    OR120 pix only OR120 120387 1972
    OR120 pix only OR120 120578 (cab OA3421)  
    OR120 pix only, preamp tubes on pcb OR120 120596 (cab OA3421)  
    OR120 pix only, preamp tubes on pcb OR120 4229 (cab OA7390) ???
    OR80 pix only, preamp tubes on pcb OR80 80039(cab OA2453) speakers Dec '72
    OR80 pix only OR80 80211 (cab OA3191) speakers March '72

Orange Graphic - pics and text '70s
Thanks to forum member Orange at there's finally some clarity in the Oarnge serial numbers. Apparently in 1973 all amps started with 10xxx. This numbering was incremented yearly until 1979. The theory is verified by Inspection tags, speaker date codes and capacitor date codes.

10000-11000 1973 OR120 pix and text OR120 10005 (cab OA3824)
  1973 OR80 pix and text OR80 10966 (cab 4556)
    GRO100 pix and text GRO100 703 (cab 4617)
11000-12000 1974
12000-13000 1975 OR120 pix and text OR120 12842 (cab 61838)
13000-14000 1976 OR80 Reverb OR80R 13839 (cab 62089)
  1976 OR120SL Overdrive mod with 3 12ax7s OR120SL 13969 (cab 62501)
14000-15000 1977 OR80 Overdrive Reverb OR80R 14010 (cab 62192) speakers date from oct '77
15000-16000 1978 OR80 Overdrive OR80 15583 (cab 63699) inspection tag inside dates from 6/7/1978
  1978 OR80 Overdrive OR80m ?? 15681 (cab OA2134)  
16000-17000 1979 Overdrive Series II OR112M 16218 inspection tag inside dates from 9/7/79
  1979 Superbass 112SB 16315  


Hustler 125 125 1274 (cab 63050) speakers date from 19 dec 1977
Hustler 125 125 1261 Inspection tag: 16/1/78

Orange Graphic - pics and text '90s reissue ;-)

OR80 pix and text OR80 712 <1995?
OD120 OR120 1219 1996
OTR80 OR80 1621 2002

How to date amps?
- It seems that some Orange amps have inspection tags on the inside that have the inspection date. This is of course of very good way to date your amp.
- by parts: several parts in a guitar amp have dates. The most obvious part to have clear date codes are the speakers in combo amps, since they are always made by specialized firms and need to have some kind of identification including a date code.
Dating Celestion speakers
Dating Eminence speakers
Some brands of pots have date codes and maybe even the transformer serials can provide dating info.