Nandi bull is Shivas vehicle (and gate keeper, will be seen in most Shiva shrines) while Brahmas vehicle is Hamsa (Hintha) and Vishnu rides Garuda. So the opium weights figures based in Hindu mythology.
The Gears say, that the bull weights were already made in the ancient egypt, in Afghanistan and India about 3000 B.C. Nandi, the symbol of Indra and later Shivas vehicle is represented all over India but especially in south India. Furthermore the Gears say, that bull representation are rare in Central Asian, Chinese and south-east Asian art.
The Gears show in "Earth to Heaven" appendix 2 (unofficial weights and weight-like figurines) only one Nandi bull. The Brauns show in "Opiumgewichte" 9 Nandi weights, but all are much smaller, the biggest is 291g and has some similarities to Maungchungs Nandi.
Gears: "[...[ bulls [...] are rare in Burma [...] used as weights in Thailand. They are employed also to stand in front of the household shrine".
However, it's a very rare and great weight with beautiful patina, especially in this size (please see image of Jochems Nandi in forum - 1 Viss too). I think the horns are unusualy big. In all the images in the literature the bulls have smaller horns. The surface looks uneven, not like standard weights. May be it haven't been cast in lost moulds???
Who knows something more about these special Nandi weights?
Michael