Hi, Redline.
Very nice.
Here are some good pictures of the best of these early offset tube bikes that I have seen, which turned up on CABE a while ago for you to compare it to.
https://thecabe.com/forum/threads/bsa-airborne-all-original.100013/
When you scroll down, you can see where the serial number is located on one of the rear wheel slots. If you've looked there, and can't find it, I can only suggest it'zs been removed, or too badly damaged by the wheel nut (not a good place for it really).
Yours has the rare early frame (possibly damaged at the weak point at the bottom of the twin seat stem tubes, one reason they moved to the later design), correct brakes and brake levers.
You would need to check the wheel hubs, the rear should be a 'Mark X' BSA, I forget the front. If the hubs are OK, the wheels look OK.
It is missing the correct saddle, the frame 'wingnuts' for the folding mechanism (difficult to find by themselves, they are not supposed to come off!). Missing too the chainweel, arms, 'pedals', stem nut, handlebar grips.
Possibly correct lamp bracket and handlebars (do they still have the T shaped nut in the middle?)
So, A mix of good and missing, but still a rare version of the bike.
I hope that helps.
Best Regards,
Adrian