How do I find the specific version of .tax file for the that I filed with the IRS if I modeled some changes after filing?

Sorry, Turbo Tax saves each entry you make and only saves the last return you did.  So you overwrote your real original return.  You will have to take out the things you tested and changed and manually put it back to the way it was when you filed it.  You should not test in your real return.  That's one reason the Desktop program is better.  You can copy and save your return with another name to test in.

Only you would know which one is the correct one. TT does not keep copies of returns prepared using the desktop software.

Hopefully, you named each .tax file appropriately. You can get free transcripts here:

https://www.irs.gov/individuals/get-transcript

Never ever mess with an online return after you file it ... as you can see you can mess things up severely.

Turbotax online does not maintain revision history. Nor does it make a snapshot of the return that you e-File (although it may seem like an obvious feature).