More than likely its Swiss. First, determine your bikes year and model. If its post '80-'81-ish its more than likely Swiss, if its post '84-ish it may be BSA (British)

Peruse these links from my website to determine:

1. Year http://cyclespeugeot.com/SerialNumbers.html
2. Model based on year http://cyclespeugeot.com/Catalogs.html
3. Threading http://cyclespeugeot.com/Threading.html

Hopefully its BSA but if its Swiss a BSA may fit..