According to Gaius, the form of the 'legis actio sacramento in rem' was as follows:
A. Hunc ego hominem ex iure quiritium meum esse aio secundum suam causam. Sicut dixi, ecce tibi vindictam imposui. (As he said this he 'festucam imponebat').
B. (Eadem dicebat e faciebat). Praetor. Mittite ambo hominem.
A. Postulo anne dicas qua ex causa vindicaveris.
B. Ius feci, sicut vindictam imposui.
A. Quando tu iniuria vindicavisti, quingenario (or 'quinquagenario') sacramento te provoco.
B. Et ego te.
A & B to the witnesses. Testes estote.
It is, I think, universally accepted first that both parties to the action were put on a level, that the right and title of both of them was in issue and that it was not enough for the defendant to base his case on the weakness of the plaintiff's claim, and secondly that in the schema as set out above, A represents the plaintiff, i.e., the one who did not have control of the slave (or other thing claimed) before the start of the actio, B the defendant. I should like to suggest that a different hypothesis for the 'legis actio', in which both these opinions are false, is possible and more satisfactory.
Towards a New Hypothesis of the 'Legio Actio Sacramento in Rem'
Available at: https://digitalcommons.law.uga.edu/fac_artchop/390
Revue Internationale des Droits de l'Antiquite, Ser. 3, Vol. 14 (1967), pp. 455-465