The seat is likely inoperative. It doesn't exist at all in Sabre GDS for UA16 on 01NOV (not blocked, just doesn't exist at all). I've seen this before on UA and this has been the reason.
If the seat was taken or blocked there would be a . or /
Blank means 'no seat'
Code:
AUTH- 777
0 - EWR 1 - LHR
NO SMOKING FLIGHT
A D G L
BHD BHD BHD BHD
1 . . . 1
2 . * * . 2
3 . . . . 3
4 . . . . 4
5 . . . . 5
6 . . . . 6
7 . . * . 7
8 . . * . 8