So what model and year my big 4 gallon tank belongs to originally? It has double indentations on the bottom for double carb's cables.
Lightning range, UK and General export models years 64-67
I have two of these tanks, one a bit like a football, the other which is presently on the bike. They have 2 different cap opening/hinge locations, either side on like yours or off at 45° (can’t remember which orientation of 45°, but my battered tank has it like that)
Supposedly it was for the clubman bikes when they used them in production racing. Though I can’t say how much truth there is in the reason, and you’d have to find a genuine clubman model with its original tank to be sure. But I do have both types and I have seen many of one but only a handful of the other and never any other variant of cap hinge position.