At the risk of bifurcating the discussion, another puzzle I'm interested in is why there were so many changes in case numbers in the early 1970s. It seems that many cases that appear superficially identical have completely different case reference numbers in 1969 and in 1971.
You can easily see what I mean if you compare the Japanese calaogues from 1970 and 1972, e.g.:
1970
http://nakahiro.parfait.ne.jp/catarog1970/moji/longines1970.html
1972
http://nakahiro.parfait.ne.jp/catarog/moji/longines2.html
It's almost as if they decided to ditch most of the reference numbers and assign new ones for no obvious reason. As a result, there is considerable confusion when people are trying to find the reference for a particular watch and haven't actually opened it to look at the case number.
For example, the steel c-case watch that has the reference 8071 in the 1970 catalogue seems to be the same as the one in the 1972 catalogue that has the reference 8317:
Another example is particularly relevant at the moment, because it's the vintage model on which the design of the recent Ultra-Chron Classic re-issue was based. This case design seems to have been one of the first Ultra-Chrons released in 1967, when it had the reference 7827 (for the steel version) and 7826 (18K). Then in the 1972 catalogue, apparently the same cases are now 8309 and 8308.
I'm very aware that minor changes to a case's design can often result in it being assigned a new case reference number. In my blog on the Ultra-Chron "jumbo" models I identified the switch from the case references 7950, 7951 and 7952 to the references 8348, 8301 and 8302 with a significant change in the style of the caseback:
https://www.moonagewatches.com/post...ls-part-2-case-variants-and-reference-numbers
But for most of these case reference changes there is no obvious change in the physical design, and the fact that they all seem to happen at the same time (1970 or 1971) suggests that there's another explanation. Unless perhaps there was a universal minor change made to all or most cases around this time that's difficult to see, e.g. in the type of steel they were made of, or the method of manufacturing...?
The upshot of all of this is that if you are interested in Longines models that were produced throughout the late 1960s and early 1970s, more often than not each identifiable model has two different reference numbers, one for examples made before (and during?) 1970 and one for those made after 1970. But why...?
