MoPac’s 50-ton twin hopper cars were essentially of two groups. The first group comprised of 750 cars in series M.P. 58000-58749 came with outside posts, 30’-6” inside length, and 1880 cu. ft. capacity. The first 250 cars 58000-58249 were built by Standard Steel Car Co. in May 1925 as Maxend cars having side outlets at each end. A photo of 58074 appeared in the 1928 Car Builders’ Cyclopedia. In 1934 the DeSoto shops converted them to conventional twin hopper cars. 
The last 500 cars 58250-58749 were conventional 50-ton hopper cars of the same dimensions with outlets between the trucks. These were built by American Car & Foundry Co. in July 1926 (58250-58499) and Standard Steel Car Co. in April 1927 (58500-58749). While similar in overall appearance to U.S.R.A. twin hoppers having the same general dimensions, there were some subtle differences including the design of the side sills between the bolsters and ends. In-service photos of these cars have been elusive. A builder photo of 58679 appeared in the 1928 and 1931 Car Builders’ Cyclopedia. 
In 1935-1936 MoPac rebuilt 425 of the 750 cars with panel sides that increased the cu. ft. capacity to 1940. I have seen 3 photos of the panel-side cars including 58308, 59492, and 58672.
The balance of MoPac’s twin hopper cars were 2,200 A.A.R. 50-ton design having offset sides, 33’ inside length, and 2191 cu. ft. capacity numbered 58750-60949. Built from September 1936 to August 1941 in 5 groups of 400 or 500 cars, they came from 3 builders (ACF, Bethlehem Steel Co., and Mount Vernon Car Co.). I covered these cars in my series of 50-ton A.A.R. hopper cars in Railway Prototype Cyclopedia with the M.P. cars shown in Volumes 9 and 25 (both books are out of print).
Ed Hawkins