Jim,
The ampersand was dropped in the 1930s. This had to do with the AAR (requirement?) that got rid of them as a standard: so C&NW became CNW etc. All the "modern" Monon freight cars delivered from 1940 on had just C.I.L.. I have a photo of a 3500 series single sheathed car, built in 1912, which still has the ampersand in 1942, so it was transitioning then. I don't know if the C.I.L. lasted long enought to loose the periods as many reporting marks did. The covered hoppers delivered in 1953 still had periods between letters.
Regards,
Mike Aufderheide