...or perhaps an old 'Amac' or 'B&B' drawing for one screw was never updated ? None of the old dull chrome screws that I have feature a cross-hatched knurling.
I am surprised that you purchased a new Amal at that price five years ago as I buy off them at wholesale price which is a World wide wholesale price and five years ago I would be loosing lots of money at that price. Like everything else over the years they have gone up in price on a yearly basis. May be the dealer got it wrong and charged you for a MK 1 Concentric carburetter?
