Go here...
https://www.backyardchickens.com/th...uckoo-barred-project-and-genetic-dis.1153068/
This will take you back to the beginning of creating the birds also known by some as "opal" legbars.
The final product is an autosexing bird with lavender but not a solid lavender bird.
You need the e+ pattern to get the auto sexing.
You could go a different route using a lavender Orpington and get the same pattern in the end but a more meaty bird then the "opals".
If interested I can explain how you could get there.