Mother cash were made of tin, or lead. Tin was preferred as it was less likely to sink into the mold from its own weight. The process was sand casting, but not with sand. Crushed sifted mud brick was used, with other ingredients to make it adhere to itself properly but not stick to the coins Weight of the coins was controlled by weight of the mother cash, each one precisely trimmed and weighed on a scale. Variability of finished coins resulted from inexact ability to close and bind the mold trays together with perfect pressure. If too loose, or if the mold tray bundles were jostled too much, the coins came out slightly too thick.
Weight standards were given in qian (same unit as mace) which is 3.73grams. The standard at the beginning of Yong Zheng was 1.40 qian. In 1734 it was reduced to 1.20. That is an unusually big change at one time, although there was warning it was coming, so some mints incrementally reduced weights starting a year ahead. Not Sichuan. Obviously such a weight reduction causes the old coins to be driven from, or recalled from circulation.
Regarding "0.12 ounce" that is the Chinese ounce, the liang (same as tael) of 37.3 grams. The standard of 1.20 qian lasted until 1852, but with multiple actual breaches below that in the interim. Generally, cash coins are found averaging slightly under official weight, with a good +/- 10% variability. Some mints 20%.
Also during Yong Zheng, and many other times, some lower weight cash were deliberately produced for alternate purposes, such as export to a part of China using lighter cash, or to Vietnam. This Sichuan production was small and at that time for local use only. This particular coin turned up in a bulk lot originating in Indonesia. Such a heavy cashpiece as this would not escape the melting pot, in China, very long. It escaped notice of the dealers handling it because it resembles those of the Board of Revenue.
I have another piece of the 1733 pattern but with diameter reduced by lathe, as if clipped. The weight is properly lowered but diameter is still too much for even 1734 or any later time. So I think the trimming was done at the mint to its stock of unissued cash. The 1732 pattern is rare enough that I've never seen one. The reduced weight Sichuan issue of 1734-35 is also rare. I have one but it's kind of ugly with blotchy patina.