Here's a nice original 1924 Transition in the upper 701xxx serial range. The final inspection is the first WTG that is not circled. Beginning about 702xxx +/-, the WTG monogram was changed to include a circle around the letters.
This pistol shows why that change was made. If you look closely at the final inspection monogram, you'll notice the lower right side of the "G" is broken off the die. I'm sure they added the circle around the WTG to reduce the stress on the die. The circle around the letters would spread the stress over a larger area and serve to prevent parts of the die from breaking off.
Shown here are a couple images of what the markings look like on an original 1924 barrel.
The K on the bottom of the barrel