The soundbox bears dark lines in geometric shapes - circles and diagonal lines. The scheme is comparable to the harp shown on the Keills cross slab and also to the soundbox of the Queen Mary harp. A detailed drawing and analysis of the soundbox decoration was published by R.B. Armstrong in 1904. (click the image to enlarge)
Armstrong reported that he was able, by looking at an angle, to see the original lines because they are impressed into the surface of the wood, while later restorations are just painted on.
His detailed drawing of what he thought were the original lines surviving shows how damaged the surface is - many areas bear no traces of the original decoration.
Most scholars (including myself) have repeatedly asserted that these soundbox lines are burned with a hot iron. However I am starting to doubt this assumption now. It is not clear how these lines were made.
The soundbox also bears the brass string shoes. These are delicate horseshoe shaped with tri-lobed ends nailed to the box. The ends are joined together so they are actually loops not open horseshoes. They may be lost-wax castings. Some early scholars state that they were gilded.
The soundholes, especially the lower left, show traces of having previously had decorative mounts around them.
At the top of the soundbox, above the highest string shoe, you can see that there is the remains of a semicircle. In this position on the Queen Mary harp is carved leaves matching the pillar. On the Trinity harp, I think this semicircle originally continued up above the edge of Armstrong's drawing, to make a complete circle. I think they were cut away to make a vertical step around which the metal band was fitted. Nowadays this whole area is covered over with modern filler.