Atlanta-based Delta Air Lines is reaching the final stages of combining operations with Northwest Airlines, more than three years after closing its deal to acquire Northwest.

Although most of the work to combine operations has long been completed, some labor issues from the 2008 merger have lingered.

In its latest milestone, Delta said it has reached a deal to combine stock clerks' seniority lists. The deal marks the final seniority list integration agreement to merge the Delta work force with employees who came from Northwest.

The 900 stock clerks are non-union, after voting against union representation in elections in 2010. The results were upheld in a decision by the National Mediation Board last November.

Some other steps remain to complete the combination of the two airlines' operations. Flight attendants from Delta and Northwest haven't been able to work the same flights together, and will begin doing so in May.