Interarms had various sources for their mauser-type rifles, including the former Yugoslavia and Spain. They may have used different standard twist rates.
It's easy enough to verify the twist rate for yourself:
Use a cleaning rod with a tight patch on a slotted tip (to prevent slipping) and insert into the more. Mark the rod with a felt-tip pen.
Use a piece of masking tape to mark your cleaning rod, and push (or pull it out enough for one complete revolution. The distance between your starting and stopping points on the rod is your twist rate.