The three coins below are three different types. One is a Meiji nibu, a common piece worth around $250 in high grades, usually less. Two are significantly rarer and worth $500 or more in high grades. Everything you need to ID them is visible in this picture. Can you tell the difference?
Don't worry, it's a subtle difference. However, it's also very simple to tell them apart once you know what to look for.
The three types we're looking at are:
Man'en Ansei-style nibu - "Extremely rare" per Hartill, only 10 graded as of October 2020
Man'en Meiji-style nibu - "Extremely rare" per Hartill, almost 100 graded as of October 2020
Meiji nibu - "Common" per Hartill, over 500 graded as of October 2020
Hartill clearly shows how to distinguish between Man'en and Meiji, though he doesn't note any difference between the two different styles of Man'en nibu.
If the top right stroke of the bu character (分) curves upwards, it's a Man'en nibu. This is referred to as "hane bu." If it's straight or angled down, it's a Meiji nibu.
To see the difference between Man'en Ansei-style and Man'en Meiji-style, we have to look to the JNDA.
This entry is labelled in English as the Man'en nibu, and it also reads "hane-bu," reinforcing Hartill's criteria for identification. It goes one step further; the caption on the top illustration (with the blue dot) reads "Ansei-style." The bottom illustration (with the green dot) reads "Meiji-style." These illustrations refer to the bottom part of the top flower, circled in purple. The example shown is a Man'en Meiji-style.
Below we have an Ansei nibu, both Man'en types, and a Meiji nibu with the identifying characteristics circled. The Ansei nibu is easy to differentiate for its larger size, but the size, weight, and composition of the Man'en and Meiji nibu are identical.
The two Man'en can be distinguished from the Meiji by the bu character. The Man'en Ansei-style is missing the small V at the bottom of the top flower, like the Ansei nibu. The Man'en Meiji-style includes this V, as does the Meiji nibu.
This is the current market definition of the different types. There is some discussion among specialists regarding whether these are correct. Most surviving examples of the small size nibu (Man'en and Meiji types) are tome bu, the type currently deemed Meiji. However, the original mintage of Meiji nibu was much smaller than that of Man'en nibu. One would expect far more Man'en nibu to have survived, but with the current attributions, this is not the case.
At the same time, the evolution of the V in the top kirimon seems to support the current hypothesis; the Ansei nibu has no V, and it was originally left off when they switched to small nibu, giving us the Man'en Ansei-style nibu. They also kept the same style of bu character. Somewhere along the line they added the V in the top kirimon, leaving everything else alone and giving us the Man'en Meiji-style nibu. Finally, the style of the bu character was changed, resulting in the Meiji nibu. It makes far less sense when you rearrange this order.
Regardless, these are the definitions that the Japanese market currently operates on, and until other evidence arises, they will continue to be. All three small types are needed for a full PCGS Registry Set, so it's important to know what you're looking at if you're going for a full set.
Use extreme caution when buying any nibu labelled as Man'en. These are very frequently misattributed even by PCGS. I've never seen a graded Man'en nibu above AU58 that was correctly attributed, and I own the only correct AU58 I've seen. It's particularly problematic for the Man'en Meiji-style nibu. I estimate that of the roughly 100 that have been graded, only about 10 are correctly attributed. Of the 10 Man'en Ansei-style nibu graded, I believe 5-7 are correctly attributed.
This also presents a cherrypicking opportunity. In February 2020 Stack's Bowers sold a lot of two nibu, one of which was a Man'en Ansei-style that was labelled as an Ansei nibu. The lot sold for $312. The Man'en piece popped up at a Japanese auction company in October, now graded PCGS AU50 and correctly attributed, where it sold alone for about $460.
The moral of the story? Always check your nibu, both raw and graded.