Ideally you would have documented your qvw files and just look in your documentation to find out this information. Your question indicates that you didn't. Perhaps it's an idea to create such documentation.
You won't find that information in the txt file so you'll have to examine the .qvw files until you find the one that generates your txt file. Perhaps the location where your txt file is created gives you a hint. Or maybe you can use the creation date of the txt file to get an indication. The qvw file you're looking for probably has a modification date that's close to it.
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