Restricting availability by point of sale (POS) is certainly nothing new....

Not only that but some airlines (BA included) have the ability to change the viewable availability by IATA Psuedo City code on each of the GDS systems within the SAME Point of Sale.

In other words.... they can choose to allow one agent in London to sell 6 D class tickets, while another agent can only see 1 D Class seat available. This would obviously be used to ensure 'preferred' agents who are more likely to sell higher yielding tickets would get 'preferred' availability.

For this reason i've NEVER trusted tools like KVS and Expertflyer who do not sell any tickets and I doubt therefore have access to the full inventory.