From episode #1, The Case of the Restless Redhead

The T-shaped mark is near the bottom of the picture between Perry and
Tragg. Della "hits the mark" when she pours a coffee for Tragg. The mark
is not visible on my "off the air" tapes, but it can be seen on the Columbia
House tape and, of course, on the DVD.

[Trivia][Perry Mason TV Series]