The underlined letters have been discussed in a few posts in the past. I think underlined T is one of the most frequently encountered but I have seen others. It might be a fun project to compile a photo list of known examples.
I think general consensus is that, as you have said, there were probably two (or more) inspectors with the same last initial working at the same time. I suspect that there were a few inspectors whose names have been lost to time and therefore have been "missed" by G&S and others.