Boeing has entered if first 737-800 commercial production type on its line. These are special times where learned lessons from the NG, 787 and 747 projects. They will produce seven production (delivery) models but will not be ready for delivery until the engine maker goes through its testing and Boeing tweaks its test model until satisfied or when they have found a "Seattle Nirvana" replication.
Rumor has it, Boeing may be months ahead with its Max schedule. They want a smooth roll out for the 737 unlike its arch rival Airbus who biffed it on the NEO having its engine retrenchments and step backs after it delivered its first single aisle Neo for Lufthansa. It has since re engineered its engine not meeting initial operational specs for its PW1100G needing extended warm-up time.
Boeing wants specifically a perfect first delivery to its customers with its CFM Leap engines. If they avoid those NG, 787 and 747 introductory drags, then it will have a leg up on the Airbus offering when selling the Max. The A320NEO has teething woes for its bread and butter single aisle offering.
Boeing will hang cement blocks under the first delivery models until all engine testing is complete. The importance is not only for reliability issues but its often quoted 14% fuel increase over the NG. If it does tune the aircraft successfully prior to first delivery it will avoid a similar situation Airbus is now experiencing with its own A320NEO's coming off the line. Boeing will hang fundamentally complete advanced engine technology under its wings. Boeing is ahead of schedule and has its developmental program under control for the 737 Max.