1966 A904 loses all gears overnight

Since the VB is factory stock, you cannot manually select second or third.You can only select reverse, manual low, and Drive-low. What I mean is, if you select second or Drive, the trans will default to Drive-low

As to no-forward motion in Drive-low, the only choices are the pump, the manual valve, and the over-running clutch.
The over-running clutch cannot be diagnosed except by inference when everything else checks out OK. But it rarely fails in a stock application. I have seen maybe half the energizing springs worn thru, and it still drove normal.
A somewhat common problem in this situation is a disengaged manual valve. A pressure test will reveal it.Alternatively, you can check the manual valve, by removing all the link-rods down at the trans and shifting the trans thru all the gear positions , and listening for each little click, and feeling each little notch,and finally;checking for the presence of the E-CLIP that keeps it all together.

As to manual low and reverse:
The common denominator is the L/R band.And the adjuster is in the pan......... So you have two options;
A) put a pressure gauge on it to prove the pump is working, and
B) if it is, then pull the pan again and verify that the strut-adjuster is still engaged, and adjust that band.
C)If no pressure, put it in neutral and verify there is adequate fluid in there. If the level is correct with the engine idling in neutral, then the trans will need an overhaul.
D) If after the strut is found to be engaged, and the band has been adjusted, and you still cannot get the car to move, then you have two options;
E) drop the pan again,lol, and perform an air pressure test on the L/R servo, and
F) just get it overhauled.
G) you can do step E), at B) after the band has been adjusted and save removing the pan a second time
I think 67 was the first year of the anti-drainback valve.

That's all I got