Sitting under your first post on this thread is a set of three covers with the overprints that you have for sale on EBay.
The descriptions for the first two are good.
The third - the 5 1/2d purple merino overprint - poses an issue in that the cover is addressed to the UK not the USA. Your comment on the rate deals with the USA.
5 1/2d is the correct rate for a registered cover to the UK at that time. (3d registration + 2 1/2d for a standard letter to a British Empire country = 5 1/2d).
If it were addressed to the USA the standard letter rate would have been 3 1/2d plus the registration of 3d = 6 1/2d.
Thought you'd be interested.