Fitted from the first coil engine in 1938 and in production until early 1931, was the CAV DS4. This had a deep body in cast iron with the points buried inside it, a large external condenser and 'push-in' HT leads.
From 1931 onwards, until the auto-advance DK4 in 1934, was the Type DJ4; this had a shallow cast-iron body topped by a brown bakelite plate that held the points and condenser. The brown HT cap was fitted with matching, domed, HT lead retaining screws.
Both the DS4 and DJ4 had advance and retard controlled manually by a lever in the centre of the steering wheel. Experimentation of the setting will show that if upon starting the engine kicks back, the setting should be retarded a little at a time until it does not. How far to advance the timing? Although there are published settings available, as backlash in the drive gears to the distributor and timing scatter have to be accommodated, if the car is run on a flat road and the timing advanced in small increments, it will increase in speed until the engine starts to rumble and sound rough. The ideal setting therefore is a smooth engine with the highest speed.
Here are some published links that will explain how to set the initial timing:

https://www.ha7c.co.uk/Technical/40%20A7%20Ignition%20Timing%20A10.pdf

https://www.austin7.org/TechnicalArticles/IgnitionTiming/IgnitionTiming.html

https://pub25.bravenet.com/forum/static/show.php?usernum=2099944454&frmid=5&msgid=925966&cmd=show

Information on the auto-advance DK4

https://pub25.bravenet.com/forum/static/show.php?usernum=2099944454&frmid=5&msgid=516288&cmd=show