"LB" is correct suffix for a 261 in a K truck, so yours is an early, "First Series Design" 1963 and your engine is most likely original to your vehicle.
Most likely, it seems that the 292 types came available when they switched to production of the 1963 "Second Series Design".
I would be interested in knowing more also.
My catalog list the 292 as an option for the C series trucks in 1963 but not the K series.
I wonder if anyone knows if 60-66 trucks came with build sheets and where they under the seat?
