Stock Alt/Gen and negative grounding.....

If the previous filter had no check valve, then you would not have noticed that the oil flow was backwards through the filter. Backward flow through the filter isn't necessarily bad, as it stills pushes the oil through a filter material, but any high pressure relief valve inside the filter would not work when the filter is plumbed backwards, meaning if there was a blockage in the filter, you'd probably blow the filter off the bike.

The way the Norton oil system works is it pulls oil from the tank (through a mesh screen inside that large banjo fitting), pumps it through the engine, then picks up the oil in the sump, pumps it through the filter and back into the tank. IN in the mounting bracket means in from the engine. OUT means out to the tank.