There are a couple different choke cable locking mechanisms that may be found on MGA, and originality is not yet certain. The locking device will always be found just behind the mounting thread at location of the red arrow in the second picture. Here there is a split tubular leaf spring (constricting spring). This picture shows a shaft with notches, which may have been original to the MG TD/TF cars (not yet known if it was ever used in MGA). Motion for this one is twist (clockwise I think) to pull out and twist back (anticlockwise) for upright knob to lock. Then twist to unlock before pushing it back in, and twist back upright again at rest. This one gives a step-wise locking action, not infinitely variable. But the knob will be upright when extended and locked.

When the rear spring sleeve is withdrawn it exposes two slots. One slot runs fore/aft and contains a leaf spring device that engages a slot in the shaft. The other slot runs crosswise and holds a thin crescent shaped key where the flat edge of the key runs against a flat side on the shaft. If you are missing this piece it can be made from the edge of a thin flat washer (#8 stainless steel flat washer may work). Function of the first axial leaf spring seems to be to prevent rotation of the shaft until it is fully withdrawn. Action here is to twist, then pull, and then twist the knob back upright for locking. When the shaft turns it pushes the crescent key outward against the tubular spring to allow axial motion. When the knob is upright the key drops into one of the notches to lock it in place. Again, this appears to be a step-wise locking function, not infinitely variable.



Photos above compliments of Jim Cheatham in Amelia, VA, USA.

On 1/15/2016, Mark Wellard in Australia wrote:
"The attached picture shows the shape of the original key which is 5.96mm long and 2.62mm high".

There is a different style, perhaps the most common, having a spring loaded ball detent and a groove down one side of the shaft. When the knob is straight up the ball rides freely in the groove allowing free pull and free return of the shaft. When the knob is twisted (left or right) the ball is pushed up out of the slot against the constricting spring, and the shaft will be locked in place by friction. Action here is to pull and twist to lock in any position. Turn the knob straight up to release. The knob can be rotated 360 degrees, but you should not make a habit of "unwinding" the twisted inner cable wire. This is the type on my MGA, and the one I am familiar with for decades.

The Service Parts Lists show only one choke control cable assembly number for all MGA. It also has part numbers for inner an outer cables which never changed. For the different style locking devices the inner and outer cables are different, so most likely there was only one style locking device ever used on the MGA, and one of the devices described above is not original. So far I can't prove which is original and which is not.