It's a Mk2, (see following message) as the Mk2 was the US market version with the perforated metal airbox, peashooter silencers, (etc, etc,).
The Mk2A was the European market version with the black plastic airbox, black cap silencers (etc., etc.,), although some late Mk2As were sold on the US market yours, doesn't appear to be one. The tail light assembly also wouldn't be correct for an 850 of any type.
A minor detail but the side panel pinstripes would normally be facing the other way with the single stripe to the front of the panel (and double horizontals.
[URL unfurl="true"]http://www.classicbike.biz/Norton/Brochures/1970/1973NortonLineUp.pdf[/URL]
[URL unfurl="true"]http://www.classicbike.biz/Norton/Brochures/1970/74NortonBrochure.pdf[/URL]
It has the 850 Mk3 alloy left-foot gearchange lever fitted with a non-standard rubber.
The petrol tank looks like an Indian-made copy.
Personally, I'd suggest removing the automatic anti-drain valve or at least fit a manual valve with an ignition cut-out.
