In the Moss catalogue the MGA Twin Cam oil pump shows that the thrust washer goes under the spindle drive gear.
It goes above the gear because the spindle thrust is upwards. On the pushrod engine the thrust is downwards and so the flange on the spindle acts as a thrust washer. That is why the thrust washer was added only on the Twin cam engine.
Although the correct placement is not mentioned in the Twin Cam Workshop Manual, careful examination of the engine cross section drawing shows it above the gear (magnifying glass required).
Mick