Thanks All. From what I know, My Father In-Law has had it since the early 1970's and it has never had any work done other than cleaning and service in Germany by an authorized servicing company. It was last service in 2015 and was assured that it is 100% original and for it's age in excellent working condition. It keeps time perfectly.
Me having it now is the first time it has been outside Europe. I am wearing it as I type. I put the band on it that he had on it as it was given to me.
There are 4 digits on the inner working of 1001 so I am assuming that is the CAL#
The inside of the back has two ref. numbers and they are,
168.033
166.052
There is also a very faint number off to the right of the OMEGA Symbol looks to be almost hand done but, 3319 appears to be what I can barely see with a magnifying glass that I have
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