Hello Michael, thank you.
The seller told me 1888 and I hadn't the time to look deeper into this.
Now I did a bit of research today. The cal nr. is 1617x. Date your IWC places that in the year 1886. In the book written by Tölke and King, you can read that the batch 15701-17200 was produced in 1886. According to R.Meis ( book p. 163 ) in 1888, there was a production of Pallweber movements ( 1500 pieces ).
When you look into the archives, the date of sales were sometimes not quite logical compared to the production numbers.
Cal nr. 15732 was sold 2nd July 1886, cal nr. 14549 was sold 30th October 1886 and cal nr. 16109 was only sold 8th March 1888.
I think you are right to say the production was not in 1888, should be 1886.
Maybe it stayed on the shelves for some time before being sold.