20/116xxx would be correct for a '66 atlas. I've seen about a dozen over the past year, and on some engines the '2' isn't fully stamped because of the curvature of the engine case. On my '68 title, they didn't put the slash. in '72 Norton stopped using model prefixes, as they were only building the Commando by then. This is also why there's a jump in serial numbers, with 1972 starting at 200001.
I think due to internal issues with AMC's consolidating of manufacturing to try to stave off bankruptcy, assembly of Matchless, AJS and Norton moved around. In the early '60s, Engines were assembled at Wolverhampton, Frames were made at Manchester, and final assembly was at the Plumstead works. Some engines were assembled at Plumstead, and those were stamped with a 'P'. Some think that Plumstead engines were superior, as this was where all the AMC competition engines were built.
AMC went bankrupt in 1966, and Manganeze Bronze bought the company, which became Norton-Villiers. Matchless and AJS bikes were reduced to Norton-powered machines (G15s, Model 33s, and Matchless-badged P11s), AJS 2-strokes and leftover singles. By the fall of 1968, Matchless was done, AJS was later sold off and concentrated on small 2-strokes, and Plumstead was shuttered by the summer of '69.
Norton production moved to Andover for some final assembly, with most manufacturing of engines and parts, and the balance of final assembly continuing at Wolverhampton. I know Manchester was the original origin of the isolastic frame, but don't know when production at Manchester stopped.
