I think that most heritage broiler birds were expected to be at good processing weight by 18 weeks. But anything from 16 to 20 seems reasonable. With a 5 lb. dressed bird being at the later end of this spectrum. Based on the chart below I would think that the weights aren't achieved until 24+ weeks. The idea with heritage breeds is that the meat is good at almost any age.our KATHY F4s first egg was 27 weeks - we will see if F5s improve
I don't know what quick development for meat means unless you are comparing to cornishX cross . I have never found info on the kill age for the original Del .
Maybe some one knows that info.
From Wikipedia
Approximate weight
Rooster 3.9 kg 8.5 lbs
Hen 2.9 kg 6.5 lbs
Ellis was apparently shooting for a broiler but ended up with a good dual purpose bird.