AccuSpark Ignition kit for the Lucas DM2 & 25D4 distributors.
Kit 4 is for Negative Earth. Kit 4A is for positive Earth.

What's in the kit

25D distributor Adjuster

The Lucas DM2 and 25D4 distributors are distinguished by having an adjustable advance wheel on the distributor body. Original units will also have the basic model type DM2 or 25D4 cast into the body near vacuum unit.

Web Site: www.accuspark.co.uk/car_search.html#MG

An interesting on-line forum exchange reveals some truth.
On Jan 4, 2024, Malcolm Thacker in Uttoxeter, Staffs, UK wrote:
"Not started the 'A' for a while and yet she kicked in to life beautifully today. Drew her out the lock up and left her with high tick over to charge the battery while I did some other work, after about 5 minutes she just cut out and wouldn't re-start. Petrol okay, but being on my own couldn't check the spark. (She has an old (about 10 years) electronic ignition so couldn't flick the dizzy like we used to). After much head scratching, let it go stone cold, she jumped into life again, for 5 minutes! I did this 4 times, same result each time, cut out as soon as getting hot".

After 10 days fiddling and discussion, we have the answer:
On Jan 15, 2024, Malcolm Thacker wrote:
"Quick update from AccuSpark (messaged them about the the one failing after 11 years). In normal use they expect their units to have a 5 year life span. So, if you've got one of theirs, and it's older than 5 years, get a spare".