It is my understanding that Semet-Solvay was always a part of Allied 
Chemical, even though the Semt-Solvay tankcars, as delivered, had no 
markings to show Allied Chemical affiliation.
I have a photo showing a Semet-Solvay car with a small Allied Chemical 
rectangle herald of undetermined date and a later image of a tankcar 
with a large Allied Chemical herald.
Can anyone help with rough dates as to when these markings were in use?
Thanks

Pierre, the various Allied Chemical companies that owned and operated tank cars did not identify themselves on the cars as Allied Chemical divisions until ca. 1946.  After that, though they kept their own reporting marks and numbers, the company names stenciled on the cars read, e.g.,             BARRETT  DIVISION

ALLIED CHEMICAL AND DYE CORPORATION

This applied to Barrett, General Chemical, Nitrogen, Solvay Process, and Semet-Solvay Divisions (and, later, the Plastics Division).

Eventually the cars of all these divisions were brought together into a single fleet with ACDX reporting marks and large Allied Chemical emblems, but this took place in the ‘60s and was not yet in effect as late as 1962.  At that time, all of the cars were renumbered by adding a prefix number to the existing number (e.g., 4 to Solvay Process cars, so that SPX 9103 became ACDX 49103 and 5 to Semet-Solvay cars, so that SSLX 3132 became ACDX 53132).