I guess you don't have to use the lock offs if your seat belt locks so i was wrong about that. I have always just used both lock offs, If i am correct they said to use both and then changed it to only one.
Per the Britax site about the change. So i'm not loosing my marbles the manual i had with my old seat did say use both! Going to only use one from now on.
A. In the spirit of continuous improvement, BRITAX has revised the lock-off installation method for all BRITAX U.S.A. next generation convertible car seats (ROUNDABOUT 55, MARATHON 70, BOULEVARD 70, BOULEVARD 70 CS, ADVOCATE 70 CS), allowing you to use only the lock-off closest to your vehicle seat buckle instead of requiring you to use both lock-offs. This change applies to both rear- and forward-facing installations and eliminates the occasional convenience button interference issues that have been observed. This change only applies to U.S.A. car seats; all Canadian next generation child seats manufactured on or after August 1, 2010 were produced utilizing the single lock-off installation method.
The change was implemented in production on October 5, 2010. All seats with a manufacture date of October 5, 2010 or later should be installed using only the lock-off closest to the vehicle seat buckle. A kit can be requested free of charge for seats manufactured that instructed use of both lock-offs. To request a kit, contact BRITAX Consumer Services at 1-888-427-4829 with the serial number and manufacture date of your seat.