There are a lot of differences between the 156c file and the POSM file not related to POSM: I don't think it is based on the 156c file so is difficult to compare/merge.
You're correct, it's not. Per the documentation, those are the (unsuffixed) zc156 version of the files, to be used on (unsuffixed) zc156 installations only.
There are a lot of differences between the 156c file and the POSM file not related to POSM: I don't think it is based on the 156c file so is difficult to compare/merge.
Same thing for the catalog functions_lookups.php