To me it just looks like an ordinary registered cover. Assuming that they are soft paper, the date is long past the earliest known, so unless it's the earliest known use of the Norwich cancel or the backstamp I don't see anything unusual.
I think the cover is interesting because it is franked with two different denominations of CBNco printed stamps on soft paper. If the stamps were off-cover then they would be identified as 184 and 188 instead of 158 and 161. The cover was also posted very close to the consolidation of CBNco with ABNco which is nice also.