I'd just go for a matching pair, without worrying too much about the year; as you will have seen from the discussion, very few people know what your bike should have. And those that do wouldn't approve of the 18" wheels, the crankcase breather, and the rearsets. I think its something like 2% of Norton owners try to keep them original.
Mine is a 1970 fastback, now interstate, and when I need to replace something I try to go for the best that is available, which is rarely the original 1970 part. When my speedo or tachometer fail, I will definitely go for these
https://legendary-motorcycles.com/product-category/gauges-brackets-speed-rpm/ . Not original, but they seem much better.