If you have other tools that can understand this file format, try to make small changes to the file through these tools (change a password but keep it at the same length, change a password to one with a different length, change a username, add a user, delete a user, ...), and calculate the difference that it makes to the file.

That should give you some idea what parts of the file stand for what. If some part of the file is always changed, it could be a checksum or a timestamp. If it's always increasing, it's most likely a timestamp, if it changes seemingly randomly, it's most likely a checksum.