Inspected by #1.
That's my guess, anyway.
Number-in-a-circle often indicates the date of manufacture (one typical arrangement is a 2-digit year in the center, with an arrow or other marker at one of the 12 "hour" positions around the perimeter to indicate the month). Looking at photos online, though, I see that these chairs were usually stamped with the Summit Prime number-in-a-circle AND a separate numeric date code.
SUMMIT STAMP
I also tried to find out what the numbers meant in the stamp, and found this image that was a lot clearer then what was on the bottom of my chair. Which was all smudged. Hope this helps. I found out that the top letters I thought were #'s, actually said US. I guess the number in the center is a 0. Not sure.
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