As of this time, I can account for 681 RailWhales, 462 6-axle cars and 219 8-axles cars.  These cars can be broken into ten ownership groups, as shown here:

ORIGINAL OWNERSHIP GROUPS
MARK	CARS	OWNER
ACFX SHPX	93	The Shippers Car Line Division of ACF Industries
CELX	101	Celanese Corp.
DUPX	223	E. I. du Pont de Nemours & Co., Inc.
ESMX	34	Essem Corp.
GATX	113	General American Transportation Corp.
IAPX	5	Industrial Air Products
MCPX MONX	37	Monsanto Company
NATX	57	North American Car Corp.
PRR	2	Pennsylvania Railroad
RAIX	20	United States Leasing International, Inc.
UTLX	1	Union Tank Car Co.

The Shippers Car Line 6-axle cars originally had SHPX reporting marks, but these were subsequently changed to ACFX. 

The Essem Corp. (ESMX) and United States Leasing International (RAIX) were both affiliated with Union Carbide Corp.  In about 1984, the 32 active ESMX cars received the Union Carbide CCBX reporting mark.  Around 1988, 25 of the 32 active cars received the Vista Chemical Corp. VICX reporting mark.  In 1998, Vista Chemical became CONDEA Vista and the VICX cars and reporting mark were acquired by Union Tank Car Co.  The RAIX reporting mark was reassigned to Union Carbide in 1981.

Most of the Monsanto cars wore MCPX reporting marks through their existence.  Three cars (23034-23039) had the MONX reporting mark for a few years, but this was changed to MCPX.