Complex sentence: SPAD designer Louis Béchereau initially produced the S.XII, armed with a single Vickers machine gun, and finally the S.XIII which waqs armed with twin synchronized Vickers machine guns.
Split into simpler sentences: SPAD designer Louis Béchereau had initially produced the S.XII, armed with a single Vickers machine gun. It was followed by the SPAD XIII which was armed with twin synchronized Vickers machine guns.