All speculation IMO.

1) YB means Cosworth part number and NOT Ford. Therefore they must pay royalties on both or indeed buy from Cosworth

2) The rings are alloy and wouldnt corrode with water and anti freeze anyway.

Ive heard these stories so many times its untrue and from various sources. I believe the YB1261 is better for one reason and one reason only. Its the last gasket that FORD themselves fitted and i believe they would choose the best considering each would cost them the same.