I bought a exhaust pipe for my '45 NG from Feked and the curve is spot on.
Very satisfied with it.
compare pictures on their website:
https://www.feked.com/ariel-ng-nh-350cc ... -pipe.html
The kind of exhaust you would get from Armours looks more like a WNG type, see website.
https://www.armoursltd.co.uk/product/a4 ... 1936-1952/
from Feked :
A typical WNG exhaust (dont know who made it):