Don't mean to sound like a crusty all know it all, but that Amal GP is actually a TT which is the prior model. The TT had the needle down the middle like every other carbs, but on the GP they moved it to the side so it would interfere with the flow.
Had a GP on my 350 Goldie and that bike was pure cafe race back in the day. Clip ons, rear sets real Goldie silencer, AVON top half race fairing, glass tank etc. When i sold it back in 74 when I was moving to OZ, I could barely get 200 quid for it. Now they are worth a fortune. I replaced it with a Rocket 3 so I shouldn't complain.