The detail is lost in the enlargement IMO. In the original, it might be TX or TA. It still looks to me as if a juvenile, proud of his 'art' couldn't resist putting his initials somewhere...btw, the aircraft looks as if it has been traced from a brochure perhaps? Tracing is what young folk who can't draw are inclined to do. This might account for the fact that the 'bridge' is so poorly drawn in comparison to the aircraft. (for what it's worth.)
If you can get more detail out of it help yourself.