Would it be possible for you to provide a screenshot? I suspect that it would be some form of parrying dagger, which was a small blade wielded in the off-hand by a duelist while utilizing a sword in the other hand. Designs varied, but some would have smaller prongs, intended to catch the opponents sword, which would certainly resemble a Sai. They were sometimes (but not always) used in duels during that period, so wouldn't necessarily be out of place, although perhaps it would be strange for that to be his only weapon. Why not a regular blade? Perhaps just an aesthetic choice by the director. They pretty much fell out of favor by the post-Renaissance period, so you don't see usually parrying daggers used in duels from the 1700s onwards.