Looks like the local flaps parts book show it in 1963.
The 261 engine was limited to the k10 and k20 (4x4) trucks.The 235 was also in the 4x4 trucks and other applications as well.
c10,c20,c30,p10,suburban 1/2 and 3/4 tons.

1951 3100 Chevrolet
1951 Chevrolet Suburban Carryall
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