Re:
Don't see any reason why it wouldn't be a Harry Hall - decals and paint are clearly pretty old and there wouldn't be any reason to put fake ones on would there?
But, just a couple of points:
1. I believe that Harry Hall did build their own frames, but I'm also pretty sure that they re-branded frames too, just put their own paint and decals on a frame of another builder.
I think that was common practice for shops who sold their 'own' frames and guess it would have just been to meet the demands in a day when people bought a different kind of frame to these days, and the frames would, no doubt, have all been of good tubing and good workmanship. But... if you were really bothered about finding out who made it, that might be impossible. Having said that, whoever built it, it was probably always badged up as a Harry Hall.
2. I knew someone a few years ago who DID find the Harry Hall shop helpful with looking up old frame numbers. You won't lose anything by getting in touch with them, if you haven't already tried.