Uli, your tank is the early version of the 17 liter; "double wrinkle" and raised area for the short neck fuel tap. relatively uncommon part.
perhaps over time we can begin to establish some consistency with the codes on "sandcast" tanks.
the first thing i would make note of, is the position of the "." (dot.) by that i mean, are there always the same number of characters in front of -or- behind the dot ? are letters always before the dot or only after the dot ? ......or both ?
are there tanks with numbers only and no letters ?
can we identify a tank with a double digit suggesting a 2 digit day of the month ? tanks used on sandcast bikes "should" all have single digits (of a single letter) denoting the month....
using as one example the tank on 2157, it would seem a safe assumption the "4" denotes 1969 when applying the Showan calendar dating system. the "4" is followed by a dot, followed by a "6" and a "4" again making an assumption the "6" = June and "4" = day 4 of June. Does this mean, at least on tanks, the numeric sequence is always year, month and date ?
is the code an inspector mark after being tested for leaks......? would all agree, we can assume there was more than one person inspecting tanks..... did some inspectors use numbers and some inspectors use alphabet letters ? Would the factory allow this variation in stamping practice......? (seems the factory would not allow inconsistency in inspector code practice, but how would we ever know.....
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why would there be a alphabet letter used instead of all numbers ? is the alphabet letter "always' in the "month position?" if so, then the alphabet letter would be "A" through "I" (and tanks manufactured -after- September would be lettered "J" and higher.....
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what would stampings look like in the case of a 2 digit date, for example, 4 . 6 10....... ?
Anyway, what a great thread with great participation. Great thread started by KP ! these codes and the distinctions seen in same parts of various parts has always intrigued/excited me.