In July 2002 Michael O'Leary emailed me a listing he had of the various Pickett Model 4 slide rules. One year later, July 2003, he sent detailed photographs he had taken of many of the Model 4 Pickett slide rules. This comprehensive information from Michael was combined with the details shown on the Author's slide rules in order to complete the Pickett listing. Please review the listing of the Pickett and Eckel slide rules as I was confused in trying to sort out the different changes of Logos and Name descriptions on what appeared to be identical scale layouts. Some changes have been picked up but others may have been missed. For example, in the change to ES & T models the three cube root scales were retained, but two of the cube root scales were placed together, top and bottom on the same scale. So the three cube root scales were shown as two. The same for the two square root scales that were combined on top and bottom so that they were shown together on one scale. Also the eight were all redisignated as LL scales. The eight LL scales ( + and - ) were then combined into four scales so that they were shown together on one scale. Also, the eight N scales were all redesignated as LL scales. The eight LL scales (+ and - ) were then combined into four scales. After these changes, other modifications were the addition of a L scale, and then later a Ln scale was added. Other hard to follow changes were additions at times to the gauge marks on the scales. I wonder if all of the variations have been listed. Also, around 1949, it seems there was a move from Chicago to Alhambra, CA. Most rules seen said Chicago on them, but there was one with Alhambra. Did the scales on these differ?