The following info is from Thompson, "SP Freight Cars vol 3", pp51-79.
(1) Of the original 2000, 1095 were running in 1950. The numbers in the book, Table 4-3, match those in the 7/50 ORER.
(2) Only four cars remained in 1955. Since K brakes were outlawed effective '53, I suspect the cars were retired rather than refit them with AB brakes. In answer to your question, I'm guessing but I would bet "yes".
(3) SP 66800-67999, T&NO 39660-39959 and 58400-58462.
Al Brown, Melbourne, Fla.