mantonas,
I hadn't noticed the earlier thread regarding this subject when I posted my previous response. It looks like you and Mike already found the reference I provided above. I did find one more piece of information from AISC Design Guide 15 (free for download for members at AISC.org).
Section 1.2 says: "The only standard referenced by AISC for steel pipe, A53, was first published in 1915. Only Grade B is included in the AISC specification." It goes on to note that A53 has Fy=35 ksi. So perhaps they based the tables in the 5th edition on A7, since it had a lower yield stress.
Can a sample be taken of any of the existing columns to test for yield strength?