Pliska 5
Inscription Pliska 5 was incised on a stone from the cornice of the apse. Researchers who found it believe that it is a numerical sign used to mark the stones. Other pieces of the same cornice have similar markings. Perhaps numerical graphs were used to record the position of each stone (Aboba-Pliska, Albume 1905).
Publications:
Aboba- Pliska, Albume 1905: Абоба-Плиска, Альбом к X тому Извѣстiй рускаго археологическаго института въ Константинополѣ, Вѣна, 1905;