It was to mark that the import tax had been paid on a particular watch. A Portuguese tax officer could easily spot a watch with or with-out the hallmark ding and then act accordingly.
"Watches imported into Portugal from 1938 to 1985 were stamped with a distinctive hallmark (marca de contraste or just contraste) depending on the case metal. The law regulating the hallmarks actually dates from 1932 but it was only fully implemented in 1938. These hallmarks were applied manually, via a stamp (punção), during the importation process, at one of the regional Assay Offices (Contrastarias), which operated at the National Mint."
https://www.watchfreeks.com/attachments/portuguese-hallmarks-pdf.135238/