Journal of Archaeology in the Low Countries 1-1 (May 2009)Stijn Heeren: New views on the forfex of Virilis the veterinarian: shears, emasculator or twitch?
2 Twitch or castration clamp?

2.5 Weight of the iron pincers

An argument against an interpretation as a twitch is that the iron objects are too large and heavy. However, the twitch could have been held by the owner of the horse (as is custom today), so not all the weight of the object is on the horse’s head. Furthermore, the object in figure 5 (right) shows a slit, which could be used to attach the object to the horse's bridle, so the weight is not dragging on the lip. The same argument can also be brought forward against the use as castration clamps. The clamping of the scrotum of cattle should be maintained for several hours. Given the weight of the iron objects, that would be really painful. No doubt this is one of the reasons that the ancient texts mention wooden clamps only, and that the surviving clamps are also from wood (Fig. 6).

In my opinion, there is no good reason to assume that the objects under discussion have ever been used as castration clamps. All available evidence points to the interpretation as twitches, used to sedate horses.