If they are txt files, they can be named anything with any extension. What equipment (data collector + software) are the surveyors using? It almost looks like .x01 is day 1 or data dump #1, .x02 is day or dump #2 and so on. You may be able to check by opening the files in a text editor. I assume you have imported points before and know the PNEZD format that the data should look like when you open it in an editor. Different data collectors create different file combinations.