I noticed that many lenses, cameras and accessories of the Olympus OM line have some white four letter "codes"; e.g. on the back of lenses, in the battery compartment of flashes, in the film compartment on cameras. Based on a comparison of these codes with the serial numbers and putative production dates (e.g. for lenses which were introduced after 1983), I interpret a code like "TNDA" as follows: letter one and two code for the production plant, the third letter stands for the production year with 1981 arbitrarily set as 1981 (i.e., C=1983, D=1984, ... V=2002), and the forth letter indicates the production month (1=Jan, ...9=Sept, A=Oct, B=Nov, C=Dec). I have not found any A's or B's in 3rd position indicating that this code was introduced in 1983. I would now like to know to which plant the first two letters refer. Based on the large number of items baring a TNxx code, and knowing that Olympus opened its Tatsuno plant end of 1981 which -for that time- was highly automated, I think it is safe to think that "TNxx" stands for Tatsuno. In a shareholders report, I found that Olympus had the following plants in 2001: Ina, Ina-shi, Nagano; Tatsuno, Tatsuno-machi, Kamiina-gun, Nagano; Hinode Plant, Hinode-machi, Nishitama-gun, Tokyo; Technology Research Institute, Hachioji-shi, Tokyo; Okaya Olympus Co., Ltd., Okaya-shi, Nagano; Olympus Opto-Electronics Co., Ltd., Tokyo Plant, Chofu-shi, Tokyo; Olympus Opto-Electronics Co., Ltd., Aizu-Wakamatsu-shi, Fukushima; Shirakawa Olympus Co., Ltd., Nishigo-mura, Nishishirakawa-gun, Fukushima; Olympus Opto-Electronics Co., Ltd., Kuroishi-shi, Aomori; Sakaki Olympus Co., Ltd., Sakaki-machi, Hanishina-gun, Nagano; Mishima Olympus Co., Ltd., Nagaizumi-machi, Sunto-gun, Shizuoka; Ohmachi Olympus Co., Ltd., Ohmachi-shi, Nagano; Olympus (Shenhen) Industrial Ltd., Shenzhen, China; Olympus Winter & Ibe GmbH, Hamburg, Germany. From the similar phonetics, it might be that OK stands for the Okaya-shi plant; NI for Nishigo-mura plant; SA for Sakaki plant. Olympus might have had a number of other plants in the 1980th. So far I have found the various codes on the following OM equipment: TNxx: lenses 8/2.8, 21/2, 21/3.5, 24/3.5sh, 28/2.8, 35/2, 35/2.8, 35/2.8sh, 50/1.4, 85/2, 100/2, 100/2.8, 180/2.8, 200/4, 300/4.5, 350/2.8, 500/8, 35-70/3.6, 35-70/4, 35-70/4 AF, 35-80/2.8, 50-250/5, 65-200/4, 20/2 Macro, 38/2.8 Macro, 50/2 Macro, 50/3.5 Macro, 90/2 Macro, 135/4.5 Macro, 1.4X-A, 2X-A; cameras: OM-4, OM-4T/Ti SAxx: lenses only, 24/2.8, 28/2.8, 50/1.8, 35-70/3.5-4.5, 35-70/4, 65-200/4, 50/3.5 Macro SKxx: 300/4.5 NIxx: 40/2, 35-105/3.5-4.5 IOxx: 1.4X-A MRxx: flash T20, T32, T1 Power Conrol OKxx: flash F280 NJxx: Recordata Back 4 As you see, some lenses seem to have been produced at different plants (according to the 3rd and 4th digit, at different time periods). On an OM-4Ti (black), I found end of last year code "TNV7 18" indicating that it was assembled in July 2002 (!). This camera I have seen in Japan. As on some other OM-4Ti's, sometimes there is a 5th and 6th digit. On another note: on some lenses produced rather late according to their serial number, the code has been omitted. Questions: 1) Is my interpretation of the code complete nonsense? 2) If the answer to 1) is no, can anybody fill in the blanks of the production plant codes, if possible with some reference to documentation (I myself have speculated enough about this)? 3) Are similar codes found on other than OM Olympus equipment (P/S cameras, digital cameras, microscopes, microscope lenses, printers etc.)? 4) Has anybody noticed other codes not mentioned in above list? If yes, please provide full code, equipment, and if possible serial number. 5) What does the 5th and 6th digit mean (as found in above mentioned OM-4Ti; I have also seen a champagne OM-4Ti with code "TNG1 23"). Thanks for any help.