His executioners didn't want his body to be found, but amputated the hands so that identification by a fingerprint analysis could be conducted by Argentine police.
It it or was usual to amputate hands for finger print purposes. Presumably so they can do them again if they makea mistake.
The Hand off approach was heavily criticised afer the Marchioness disaster ten or twenty years ago when all 110 dead ones had hands amputated and the relatives created a huge row.