Anyone able to help this chap out?

A guy e-mailed the Spitfire replica discussion group recently with this question.

I think there was a question about this before, can’t recall when, though.

I am a docent at the Flying Heritage Collection In Arlingtion WA. USA. We
have Mk Vc Spitfire that served with the Czechoslovak squadron 312. It has a
marking that we have not been able to identify, could you help or tell me who
could?

The marking is a square about 15 -20 cm on each side with a line going from
the lower left corner to the upper right corner. In the upper left space is the
letter”W” and in the lower right space is a “T”. Some of us believe it means
“wireless transmitter” but these are located at many places on the aircraft
such as both the vertical stabilizer and rudder, horizontal stabilizer, on the
lower portion of the wing near the tips, and a few places on the fuselage. On
place on the fuselage is on a small access cover on the left side just aft of
the cockpit. It says W/T or ballast.

I have some 50+ pictures of Spitfires and only 1 has this marking and the
caption does not say anything about it. I then assume that it is not something
very common.
I am confused!! Can you help me?
Thank you in advance;
Roger Hagen