C=collect, D=drop, R=received, T=transit.
The intent was to change the service letter for processing each type of mail. The reality is that it didn't happen and the same service letter stayed in place for months and meant nothing. Finding what appear to be matching uses is usually just happenstance. That said, Chicago did use "T" in marking some mail which went through town.
Here is a card which went from Denver (with "R" within the wavy bars, in error) through Chicago (true transit use with the "T", printed on the bottom of the stamp) to its destination in Martinsville, IN.
