Dennis,
Look at the link to "The Forgotten 261" in the Tech Tips to the left. It shows where to look for the casting numbers and the Captain's bars.
The Captain's bars are not an absolute indicator, as I have a 1954 261 that does not have the bars and the casting number indicates that it is a 235. However, it is is most certainly a 261, but the head would need to be removed to tell for sure. This engine was purchased as a new replacement engine, and Chevrolet must have used the wrong external patterns when it was cast.
However, I have not heard of a "false positive" - numbers that say 261 but the engine is not.
Also, if an engine has a full-flow filter with 1/2" NPT fittings and hoses it is most certainly a late 59-62 261. Hope that i did not confuse you!
