I got an extract from zenith for a cal 133.8 and it listed the ref as 1/2000 My immediate thought was they made a mistake but who knows. Usually their references are preceded by ZH ZO other letters or just a number.

 

So I heard back from the Heritage department regarding the Reference number. Apparently this was their old system where the first number is for the case material, in this case gold (1). The second number is the model number. About 1964 they changed to the second format with material indicated by letter as in A386 and the numbers for the model.