Final answer:
The specific instrument used in the Ludi Harpastum festival is not mentioned, but references to the lyre and string instruments in ancient ceremonies suggest a lyre-type instrument might have been used.
Step-by-step explanation:
The instrument used in the Ludi Harpastum festival is not explicitly stated within the provided information. Nevertheless, the reconstructed instrument made from a burial at The British Museum, and initially believed to be a combination of a 'harp-lyre,' suggests involvement of a lyre-type instrument. The mentioned lyre from the Cycladic grave sites and the lyre being played in the funeral procession give us clues to the kind of instruments used in ceremonies and festivals during ancient times. Moreover, the Byzantine lyra, a significant instrument highlighted by the Persian geographer Ibn Khurradadhbih, represents the early European bowed instruments and could reflect the type of instruments used in similar historical contexts.
It is essential to note that while we have details of various historical instruments like the lyra, sistrum, and others utilized in religious and social festivals, the specific instrument for the Ludi Harpastum festival requires further historical specificity that isn't available in the provided references. However, the information suggests that string instruments were of considerable importance and were often central to the musical traditions of such festivals.