Control for the cold-air flap in the left-hand footwell, in the closed position. This is the later lever, longer and angled downwards giving more leverage:
The early lever just pointed sideways making it much harder to move as well as being more hidden under the dash. They also had black knobs (this is the Gaydon cut-away hence the flap itself being white instead of the usual black):
Open to the first notch ...
... and to the 2nd, you can see the flap open directly above the handle, between the rear edge of the centre console and the demister ducting:
The British Motor Museum Gaydon 'cut-away' MGB clearly showing the four notches in the control quadrant, giving one closed and three open positions. The third notch seems to be deeper than the others, which will mean it takes more of a pull to get it from that to the fourth, which is maybe why many (well, everyone else as far as I have found) thinks there are only three notches and hence only two open positions
Herb Adler's mod with a coil spring (arrowed) between the head of the bolt and the detent. This allows the arm with the roller to move downwards slightly when moving the cold air flap, rather than having to spring the arm itself, making the flap easier to open and close. A longer bolt may be required, secured with threadlock or possibly a lock-nut. This also shows one of the vanes that are attached to the back of the flap (nearest the camera, concealing the roller) which direct air sideways into the footwells when the flap is open.
The back of the cold air flap showing the vanes that direct air into the footwells and the notched quadrant:
You can just about see the quadrant and roller detent with the flap open from low down in the driver's footwell of a UK car, looking upwards. This gives the general direction:
Zooming in, shown in the third open position:
If the roller detent goes past this it will come off the quadrant and you won't be able to close the flap. The quadrant and detent are closer to the left-hand side, it's possible that with one person wielding a suitable lever from that side another person on this side may be able to 'talk them in', as it were, and lever the roller detent down and start closing the flap. The other pictures on this page should give enough info to help see what you are aiming to do.