Sorry for the delay but I just ran across this topic. I have been acquiring information about this particular car for about two years (just in case I ever finish building the one I have started). I have a few pictures and a (very rough) list of owners along with any prototype references I could find. Most of the cars were black (although I have some b&w pictures that look lighter than black) with either silver, black or red tanks.
Here's a list of car owners/numbers that I have been able to assemble:
ACFX 401 (photo)
ACFX 426 (Tank Car Color Guide Vol 1)
ACFX 440 (decal)
ACFX 441 (photo)
ACFX 513 (decal)
ACFX 522 (Tank Car Color Guide Vol 1)
ACFX series 401-441 (from 441 photo info)
Columbia Alkali
CACX 202 (book and decal)
Canadian Industries Limited
CILX 90-92 (photo)
CILX 450 (photo)
Built Sept 1936
Diamond Alkali Co.
DAX 309 (photo)
SHPX 240 (ACF Tank book)
Built Aug 1949
E.I. Dupont De Nemours & Co. (DuPont)
SHPX 220-229 (ACF Tank book)
Built Apr 1948
E.I. Dupont De Nemours & Co. (DuPont)
SHPX 216 (Tank Car Color Guide Vol 1)
SHPX 400-414 (ACF Tank book)
Built Feb 1953
Electro Bleaching Gas Co.
EGBX 208 (book and decal)
Harcros Chemicals
HCIX ? (photo from article)
Hooker Chemicals
HOKX 211-213 (book and decal)
HOKX 391 (photo)
Jones Chemical
JCIX 122 (NMRA)
JCIX 131 (Tank Car Color Guide Vol 1)
JCIX 1136-1137 (photo)
JCIX 1155 (model)
Pennsylvania Salt
PSMX 1016 (photo)
PSMX 1014-1016 (book and decal)
Olin Matheison
RTCX 340 (Tank Car Color Guide Vol 1)
SHPX 222 (photo) See footnote
SHPX 504 (NMRA)
SHPX 2110 (Trading Card)
Solvay Process Co.
SPX 854 (photo)
UTLX 80000-80003 are believed to be former CILX 90-93. Cars were renumbered in 1956.
UTLX 80003 (photo)
Built in 1945
Virginia Chemicals
VCSX 212 (book? and decal)
SHPX 222 is identified as AAR type FM; similar cars have also been denoted as AAR type TMU because these cars really don't function as flat cars for any lading except removable, high pressure gas cylinders. These cars were used to handle acid gases like chlorine for customers without direct rail service or the inability to unload and store a full tank car load. The last large-scale user of this system is DOD. Many outdated chemical weapons materials traveled their last miles in these unusual tank cars. This car has been imported in brass at least 3 times in the last 30 years and was actually a "dollar model" project in Model Railroader magazine.
Tank Car Color Guide Vol 1. Morning sun books 2010, at pages 120 and 121 there are 5 different photos of this type of car (ICC166A500: ACFX 426, ACFX 522, JCIX 131 (Jones Chemicals), RTCX 340 (Olin Matheison) and SHPX 216 (Dupont).
I'll post the rest of what I have later today.