Nice to see an original.
But it doesn't explain what it's for and why it's on so few carbs....

I assume you mean this....

Never seen that before. Have probably rebuilt close to 1000 Q-jets back in the day, but really not many 68 - 69. Most were later.

Always something to learn.

I have a 7040204 on the shelf that also had the red/orange "R" stamped on the side of the carb. It was almost nice enough to leave as a survivor, but the finish on the very front of the carb was toast--the rest was nice--wound up restoring it--reluctantly.