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Why don't use the Serial Number to get this number ? We can probably say that the number of unit built is about the number of unit sold.
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We can use the Serial Number information. Usually, an Apple IIgs serial number looks like : E749YJAA2S6000 (read as E-7-49-YJA-A2S6000).
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With
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E : Factory code (E=Singapore, NE=Singapore?, CK=Cork Ireland, C=Cork?, ...)
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7 : Year of production (7=1987, 8=1988, 9=1989, 0=1990, 1=1991...)
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49 : Week number (01-52)
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YJA : current # / based on units built this week
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A2S6000 : Apple IIgs code
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=> more info on s/n at http://macfaq.org/serial.html
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The 3 letters YJA use a base 34 system : 0-9 A-Z (10+24) where uppercase 'i' and 'o' are missing to avoid confusion with numeric '1' and '0'. Here, Y=32, J=18 and A =10. So YJA= 32*(34^2) + 18*(34^1) + 10*(34^0) = 37614 units (at least) built in the beginning of December 1987.
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So, with 7 years of production (1986 - 1993), we have to collect enough Apple IIgs serial numbers to cover the 7 years * 52 weeks = 364 weeks. How to get all these serial numbers ? First idea is to google 'A2S600' and get all serial numbers from Ebay auctions or Web sites (http://www.apple-iigs.info/collection.php has already dozen of them). Of course, asking in the newsgroup is a good option because everyone here has 2 or 3 IIgs around.
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Even if one week or 2 is missing, we can probably consider that the IIgs production was about the same during one month, so we can compute an average with the two weeks around. The idea to to find, for each of the 364 weeks, the biggest 3 digit number to have a good approximation of the maximum number units built this week. The total number of Apple IIgs built is the sum of all the weeks.
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As we wrote in the "Speak with data" sheet, Apple publicly announced 1M units sold, we are close to that with the s/n of the keyboard, and far away from that figure with the unit s/n. We may perform new unit s/n calculations based on his message.
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Antoine & Olivier