For those of you like me who have thousands of files that have to be appended with _2x1 - a search for a program to work in Windows 10 brought up BULK RENAMING UTILITY. This is a freeware product. I used their SECTION  ADD and then the append box to add _2x1 - highlighted the files to change . It works perfectly and easily and will save you a lot of time. Disclaimer - I have no connections with the company or the software.
Stereo Photo Maker (also free) as a Multi file rename in the File menu. It worked fine for me to append the _2x1 to my Fuji W3 sbs’s. If not familiar with SPM, it’s essentially a must for those doing sbs work.