Complex sentence: In 1975, the first of five DC-9-51 was implemented, and in 1977, Austrian became, along with Swissair, the first Customer for the DC-9-80 (or McDonnell Douglas MD-80).
Split into simpler sentences: In 1975, the first of 5 DC-9-51 was implemented. In 1977 along with Swissair, become the first Customer for the DC-9-80 (or McDonnell Douglas MD-80).