Hi Gaz
Strickly speaking no but they should ideally be threadded with a fine thread & metric fine threadded bolts are a real PITA to get hold of, not to mention expensive.
7/16" UNF bolts have been used for years & has more or less been adopted as the industry standard for seatbelts
If you buy a pair of harnesses new, they will most likely come with 7/16" UNF fixings ( Mine certainly did )
All is not lost mate.
You should be able to pick up a decent 7/16" UNF tap on ebay for not a lot of money
Hope that helps