What is the 53 nkat vial of thrombin equivalent to in NIH units?

Based on an assay with S-2238, for bovine thrombin,1 NIH-U = 1.15 IU = 3.4 nkat. According to Chromogenix, bovine thrombin, 53 nkat_S-2238 is equivalent to approximately 21 NIH-U or 25 IU.