filipo,
Thanks for the reply and now I see there is work for you to do.
Let's start with the Royal Purple, to give it the official title. [They named it Royal Purple because it was to be presented only to the King. How other copies 'escaped' to the open market is unknown].
There are two types of watermark which I will scan and post later when work allows. One is of multiple aeroplanes the design of which makes it nigh impossible to determine whether it is upright or inverted. The other Watermark is of three single planes vertical and in 'nosedive' position so easy to tell if upright or inverted.
The Royal Purple label will have the Multiple planes watermark and was printed on Chalk-Surfaced paper, so will be very white. It is WITHOUT gum.
So if you can check again the 7th label of the top lot and the 6th label of the bottom lot I would be very grateful. Any finding may be of importance.
A reproduction label was printed for the 1975 Exhibition at the Royal Festival Hall and is the same colour and Imperf but DOES NOT have a watermark.
If indeed you do have a Royal Purple example then the Market value is $70. However, the actual price one would expect to pay could be 3 times that amount !

Now the gold.
Slightly more complicated as there are colour variations [Gold to Gold-Brown] and I cannot tell from scans alone. If your possible gold copies do not appear at first glance to be so, then hold them up to a bright light and look for a sparkle, which does not appear on the brown copies. Check both carefully.
Apart from the colour variations there are other differences with the Gold types.
One has NO WATERMARK and is on GUMMED,CHALK-SURFACED paper.
The other has the SINGLE PLANE WATERMARK and is on ORDINARY,GUMMED paper.
It is not known which type was presented to His Majesty on that May Day in 1923. Maybe a letter to the keeper of the Royal Collection would help to solve the query ?

If indeed you do have the Royal Purple and Gold examples then you have some scarce British Exhibition pieces which should be treasured forever. Let us hope it is so. [I will be very happy for you but insanely jealous at the same time !]
If your copies differ in any way from the above, then more research and examination is required. I would like to help if I can and if it's OK with you. If they are 'new' finds then you can write to the BSE catalogue authors with info and scans and you will be acknowledged in the next edition. A nice touch.I will give you all the relevant details and who to contact. I have made a few new finds this year which have been previously unrecorded. [Not of these issues above I might add !]
So, good luck with further checks and I look forward to any more info you can impart.
What an exciting return from my Christmas holiday !

Londonbus1
PS. One more question. Do all of your examples have gum ? Are they unmounted or mounted ? Thanks.
Info Acknowledgments: British Philatelic Exhibitions by Glenn H. Morgan and Graham M. Wilson.