I first learned about the Compiled Military Service Records from Michael Strauss. He was the guest on the 82nd episode of the Research Like a Pro genealogy Podcast.
Michael Strauss talked about how Fred Ainsworth created these cards to make finding out if a person requesting a pension did, indeed, serve in the military. You can learn more about Fred Ainsworth at his Wikipedia page: https://en.wikipedia.org/wiki/Fred_C._Ainsworth.
I feel like this is a podcast that I could listen to multiple times and still learn new things each time!
If you would like to learn more about Michael Strauss, you can go to his website here: https://genealogyresearchnetwork.com.