There is also another configuration which doesn't show up on seat maps. There are only 22 first class seats; no 6A or 6B. That is why 6A and 6B are never available for advanced seat selection on a regular 757. Also there is no 19D, 19E, or 19F; that is why these seats aren't available for advanced selection and row 20 is not available for advanced booking as it is saved as "bulkhead" row.

Marc