I have just bought a Bantam (last Friday) which I am going to restore, hopefully in time for the summer.
The last BSA bike I had was a Road Rocket in 1965 when I was 18 years old, picture attached.
Starting again (had a Bantam when I was 15) on something small to see how it goes, mean me not the bike.
Attaching a picture of my Bantam, as you can see I will be looking for many parts, post another picture when
completed.
Looks like D3 front forks with larger than D1 diameter showing below the bottom yoke.
The rear mudguard looks like still has the pastel grey finish which was used on the D3.
So I think likely it was made as a D3 bantam Major.