Hello Kim!
This Jacob Månsson is my ancestor. I agree with you that something is strange about the weddingdate for Måns Johansson and Johanna Thomasdotter.
The taxrecords called mantalslängd gives some more facts. The years 1725-1731 Johanna lives as piga in the household of Johan Persson and his wife (Gertrud Månsdotter) in the farm Vishult. Johan and Gertrud are the parents of Måns Johansson (1712-1798). Måns Johansson is mentioned as their son in 1729, even his brother Anders Johansson is namned this year in mantalslängden.
As we see here - Johanna Thomasdotter was unmarried when the taxrecord of 1731 was written. Later the same year 1731 august 11 the daughter Maria were baptized.
My theory is that Måns and Johanna were married may 30, but in 1731. And their notice is written in the wrong year.
That can explain why Johanna is unmarried 1731, and the wedding would be before the first child is born. And Måns Johanssons age is exact 18 years and 6 months in the weddingday, that is more possible to belive.
What do you Kim think about this theorys?
With regards from Mats Kindahl