The rigid frame, barred tank, headlamp nacelle and alloy brake plate all make it an early 1950s 500cc Speed Twin. There was a 650 model introduced about then, I'm not sure if it had quite the same look.
You are right, those pipes are not how it left the showroom.
If those are open megaphones, you could have heard it coming from quite a ways away,
which was the whole idea !!
Great photo, is that your mum ?
And in the brochure