Here's a pic of all 3.
Early walker was just an early walker with 2 piece sear and 1 piece sear. Found on the 6 digit m40s and m40a1s. Later E prefix m40a1s had a standard walker.
The M24 trigger used in the A3 to A6 I circled the difference. It was a second weight adjustment screw adjustable through the trigger shoe. When in the stock the only way to tell its a walker or m24 is looking at the shoe to see the adjustment screw. They have gone for $300+ when you can find them.
I have all 3 and performance wise a plane old walker has the best break and weight. That said the m24s are tuned and typically have been put on a trigger pull guage and have a printout (not all) to meet certain specs. I have 1 great walker but the m24 beats the others, I'd bet most will.
Slapping a $45 walker in there will do just fine until you find an m24 if you want to be true to spec and your build calls for it. If you want a little nicer feel in your walker Wolff makes trigger springs that will give it a nicer feel.