This series of cars was built from 52 of the 60000-60299 gondolas, which were built during 1930. The bulk head flats were built in four groups: 4000-4024, 1952, 4025-4040, 1953, 4041-4045, 1956, 4046-4051, 1958. see: http://www.frisco.org/shipit/index.php?threads/60000-60299-outside-braced-gondolas.4357/#post-61660 The cars were assigned to wall-board service from Acme, TX(Georgia Pacific), and Southard, OK (USG Co), as follows: 4000-4024, 4041-4045, USG Co., 4025-4040, 4046-4051, Georgia Pacific.
Based on the photo in Molo's book, and my interpretation of the drawing... 1. Side sill 2. Solid 3. The floor extends to the end sill.
Nick Molo's book is still listed at Morning Sun Books at $59.95. Here is a link. Dan http://www.morningsunbooks.com/southcat3.html
With having to do much custom painting and lettering in G scale, Nick's book is like the Frisco bible!! Still have to compromise in certain area's, but it helps get as close as possible. ENJOY!!!! Dan