Dating japanese coins
There is variation but they tend to be on the yellowish brassy side and the file used to smooth the face is very rough, leaving clear file marks on the face.
In 1860, under grave financial difficulties the Tokugawa government began minting the coin in iron but this was relatively unsuccessful and the coins are moderately uncommon.
Bunsei coins tend to be redder and less carefully made than the Meiwa versions.
Note the rough file marks on the back of the coin, and also the hole under the tsuu character.
Most simply it can be recognized by the line curving down left just above the center hole.
This line is the katakana character "no." Within the Aizu coins there are two main types.
Of course, many domains just made counterfiet 4 mon without bothering to ask for permission.
Here is an example of an iron version of the coin which was made by Aizu domain (but within its Edo mansion at Fukagawa ward) from 1866.
These are largely identical but the quality is much moer varied, which is to say that many of the 4 mon from this era are of poor manufacture.
In 1769 the design of the reverse changed to have only 11 waves and all subsequent versions used the 11 wave design.
It was not much larger than a 1 mon coin (27-28 mm vs 25-26 mm) and so it was cost effective to produce.
This was desirable in a time when there was a growing scarcity of copper.
Because the economy itself was usually growing in this period the coins were readily accepted by users.