Amphidromus nicoi and amphidromus arlingi in my recent study, seems to be the same species, based on the recent batch that I bought currently. I am undertaking a study to understand the relationship between the various complex of Vietnamese amphidromus with the help of my friend Thai Thien, a young amateur malacologist who has a bright future in the field. Similarities:
1) Both are fully dextral species – my opinion is that there are 2 very similar congeners – nicoi here and also the servernsi (I think arlingi daklakensis is conforming to the servensi but need more specimens or maybe might prove to be a different species altogether).
2) Reddish apex, carmine red subsutural bands, spiral bands, widely spaced axial stripes at the spire stripes at spire whorls that are crossed by fine white spiral lines. – We now know that the pattern and stripes are not good indicator as this is variable within same population of amphidromus.
3) Columella rather straight, bordered by black stripe followed by another carmine red stripe. colour yellow with pink early whorls, white outer lip and columella – another non-consistent indicator for amphidromus as they are very variable even within a population. A. hongdaoae is also described with white lip but you can see in the various forms in my page, they also come in purple lip.

Nicoi and servensi seem to be closely related, and in my opinion, the arlingi daklakensis is actually a servernsi or closely related.
1) both have sinistral and dextral shells
2) Both have thicker shell structure than nicoi / arlingi

Amphidromus maxi, Thach, 2020 should be a synonym