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Identification information for the thrust chamber stamped into a burnished area on the thrust chamber's throat area

P/N 206595
S/N 4098345
U/N 2148
Code 02602
X-Ray 69AA-080

Based on a tag on another thrust chamber when I saw a group of J-2 thrust chambers in Redstone Arsenal Building 7341 back in '09, I believe "U/N" is "Unit Number".