The El Tigre Belt is the mystic item of El Tigre.
While the initial conception of the mystic El Tigre Belt remains a mystery, it is clear that it has been around far longer than earlIer Mexican civilizations. The difference of the belt from the other objects of power, is that the belt buckle needs to be spun to be activated, unlike the boots which give power by merely wearing them, and the sombrero, which can be used on mental command. It is also quite obvious that the belt is far more powerful and harbors a much more powerful spirit inside of it, the Ancient Tiger Spirit. The fact that the belt must be worn and the buckle must be spun is likely the only way of holding back the power of the belt. The only other wearer of the belt was around far before any of the other Rivera's, seen as the first Rivera, dating back before/during the Meso-american era, so it is safe to assume the belt was conceived around this time.
The actual purpose of the belt could be observed rather well. In Aztec mythology, the current universe that which they (and we) lived in was the universe of the Fifth Sun. In one of the many Aztec Myths of Afterlife, those that had not done their rightful duty during their living, would have their souls torn apart by a ravenous tiger. This was likely the inspiration of El Tigre's powers and the Ancient Tiger Spirit, as El Tigre had power over both the land of the Living and the Dead, and could travel between both, likely to do the part of the Tiger Spirit and punish those who had not done their duty to the world or wronged others.
In "The Mother of All Tigres" Manny says he received it after Maria went on tour as a singer, implying Rodolfo or Grandpapi gave it to him as a secret, especially considering Maria did not initially know about Manny becoming El Tigre.