Frame differences Series 2 / Series 3

[challenger]

Does anybody know if a Series 2 frame, with the Loop removed, is the same as a Series 3 frame with the Loop removed?
I know the rear support over the rear mudguard is different as are the outriggers for the rear runners, but is the tube basically the same length and shape?
Are the outriggers for the Legshields wider?

I’m building a new Scooter with a removable frame loop and I don’t want to cut up the only frame I have, (Italian GP 150).
I’ve been offered a Series 2 frame with a missing Loop for small money which would be better suited for the project.

I need to find out if the main frame tube remained the same between Series 2 and Series 3, I can change the Leggy outriggers to match and the back end under the panels needs to be worked over anyway whichever frame I use to get the removable Series 3 Loop fitted .

Thanks