Below are some pictures comparing them to the older 50 series.
Older 50 series (clear) on top
As you can see they discontinued the inside divider or partition on the new model, and for some reason added this one little "bump" (circled in blue below) to prevent it from being used in the older model ... which I can see would allow better airflow, and possible better "passover" mode.
If I swithch tops, the clear top on the smoky bottom, it goes in to my older 50 series, if I put the smoky top on the clear bottom, it will not go in ...