At this point I have gone as far as I want to go into the details of the Santa Fe boxcar fleet for this period. I still have some things that I haven't been able to identify but there is enough information that I can buy boxcars with confidence.
The thing I found most interesting is that the 50' cars only made up a little more than half the numbers, the 40' cars were pretty close behind.
There were approximately 7600(+) 40' boxcars still in service in '74-'75 and the bulk of that (6388) were made up of 17 types. Among these are three series that I can seem to pin down; 22232-22424, 33500-35999 and 50000-50309. The known types are (from largest to smallest): Bx-60, Bx-85, Bx-63. Bx-62, Bx-126, Bx-85, Bx-57, Bx-59, Bx-11/12 (rebuilds), Bx-136, Bx-13, Bx-115, Bx-136 and Bx-153.
There were approximately 8900(+) 50' boxcars in service for '74-'75 and the bulk of that (6955) were made up of 26 types. There are a lot more series here that I can't identify; 4100-4399, 48050-48549, 8000-8999, 48601-48999, 49501-49899, 47550-47849, 9700-9998, 47850-48409, 13500-13999, 42750-42949, 49300-49499, 4100-4399, 62000-62299. The known types are (from largest to smallest): Bx-72, Bx-81, Bx-69, Bx-145, Bx-74, Bx-75, Bx-66, Bx-70, Bx-79, Bx-154, Bx-163, Bx-61, Bx-82, Bx-64, Bx-178 and Bx-170.
I'll cast about for the missing series and see what I can come up, but the next step is really trying to figure out what boxcar types are actually commercially available. That might take even more effort since manufacturers typically don't indicate which Santa Fe boxcar type they are selling. Should be interesting.
Life is short, break the rules. Forgive quickly, kiss slowly. Love truly, laugh uncontrollably and never regret anything that makes you smile. - Samuel Longhorne Clemens (Mark Twain)