Japanese Tenpo era Mameita gin (silver bean money)

[Pharsalos]

I recently purchased this nice example of a Japanese Tenpo era Mameita gin (silver bean money). It measures 24mm x 17mm and is 10mm thick. The weight is 17.34 grams. I believe this example dates in use from 1837 to 1858 CE, though the Tenpo era is from 1830 - 1844. These are quite debased and average only 26% silver for this era. I have one other example of Japanese bean money, an older but much smaller genbun era Mameita gin.

This example seems to have a small bankers' or 'chop' mark on the reverse. I can make no sense of it; does anyone have any idea or have they seen similar marks punched on other Mameita gin?

I would also like to know how these were produced. This example is rounded like an egg, only the punched face is flat. The top half is is quite conical. The other Genbun era piece I have has a flatish rough bottom and a flat punched top, but the sides also are incredibly rounded. Both of them seem as if the sides were intentionally rounded and smoothed. Any information about these would be appreciated, I have found little information on the web so far. Thanks for looking!

[Pharsalos]

I have been given an answer to some of the questions I asked above. These were apparantly cast in sand, which I guess explains the variety of shapes and sizes they are found in. Later they were tumbled together in grit, possibly bamboo, until the surfaces were smooth. They were then stamped with the era mark. The small punch mark visible on the 'reverse' of the photo above was a banker/trader mark verifying the silver purity.