From what I understand, the “_C_Number” bit, like the “_C_29” added to your MyGameInstance class is just an iterator the engine uses on compiled versions of Blueprints and has no bearing on the actual class name for functionality, so the issue shouldn’t be with the “_C_X” ending.
It’s likely the issue is with how you’re saving and loading the data, as this system saves it to a file on disk and loads it from said file on disk instead of making it persist inside a class between game loads. Make sure you’re passing the same filename and path to both the Save and Load functions. Last I checked, the Save to and Load from Slot nodes always point to a subfolder of your project’s Saved/SaveGames directory, but leaving the filename blank (or possibly invalid) won’t save anything.
This functionality may have changed in the engine since I figured this out back around 4.17 or 4.18, so I’ll look into it and see if there’s anything I need to fix with the guide.