In Alma 17:39, the Lamanite king's servants carried the severed arms to the king as trophies of the encounter. According to Glenn Scott, this was a common custom in ancient America. [Glenn A. Scott, Voices from the Dust, p. 147]
“They Went in Unto the King Bearing the Arms Which Had Been Smitten off by the Sword of Ammon”
According to an article by John Lundquist and John Welch, the practice of cutting off the arms or other body parts of enemies, specifically as a testimony of the conquest of victims, is attested in the ancient Near East. On the extreme left of band 4 on the decorated Gates of Shalmaneser III (858-824 B.C.), Assyrian troops are shown cutting off the heads, feet, and hands of vanquished enemies. "In other reliefs, the artists of the Assyrian kings depict the military scribes recording the number of enemy dead in accordance with the number of severed heads, hands and feet which Assyrian soldiers hold up before them." This practice seems related to that of the servants of king Lamoni, who took the arms that had been cut off by Ammon into the king as "a testimony" of what Ammon had done (Alma 17:39). The reasons for this phenomenon were based on a need to obtain an accurate count of the dead, and the need to reward the soldiers involved. Ammon, of course, had no interest in receiving compensation for his loyal service to king Lamoni, but the fact that the evidence was presented to the king, which could have entitled him to payment, heightens all the more the fact that Ammon sought no recognition or reward. [John M. Lundquist and John W. Welch, "Ammon and Cutting Off the Arms of Enemies," in Reexploring the Book of Mormon, p. 180]
Alma 17:39 They . . . went in unto the king, bearing the arms which had been smitten off by the sword of Ammon . . . for a testimony of the things which they had done ([Illustration]): Ancient Near Eastern warriors often cut off the hands or other body parts of their victims and presented them to their commander as a witness of those they had killed in battle. Figure A shows soldiers at the time of Ramses II cutting off the hands of their victims, and Figure B shows hands being piled up at the feet of Ramses III. Line drawings by Michael Lyon. [John M. Lundquist and John W. Welch, "Ammon and Cutting Off the Arms of Enemies," in Reexploring the Book of Mormon, p. 182]