Byzantine Reliquary examples from the 10th century, are said to have be found in Istanbul, they are of gold and decorated with Cloisonne enamel. Such pieces were also made in Spain in the 17th century, sometimes of hollow gold set with gemstones. It loos like one large compartment on the back of this cross? Not sure. Some examples have several compartments for a number of relics.
Very interesting piece David! What I find particualry interesting:
1. The shape of the cross having eight arms; a very early example in the evolution of the Maltese cross. 2. The straight lines used in the despiction of Jesus cruxification. Straight lines "etchings" as you mentioned were also on the reverse rather than "carvings" are easier to execute on a hard metal surface than curved lines using hand tools, which indicates a more primitive hand tool was used here. I think it may possibly be earlier than 15th century.
In the 15th century, the eight points of the four arms of the later called Maltese Cross represented the eight lands of origin, or "Langues" of the Knights Hospitaller: Auvergne, Provence, France, Aragon, Castille and Portugal, Italy, Baviere (Germany), and England (with Scotland and Ireland).
The eight points also symbolize the eight obligations or aspirations of the knights:
to live in truth to have faith to repent one's sins to give proof of humility to love justice to be merciful to be sincere and wholehearted to endure persecution