Seeking advice/assistance from our resident Oceania/Pacific Island/New Zealand experts...
Pictured on the left is Penrhyn Island 1927 2½d red-brown & dull blue, listed in Stanley Gibbons as SG40, and in Scott as Sc31.
On the right is a similar stamp, but in colours that are best described as chocolate & steel blue. The watermark is identical to the regularly issued stamp. As are the perforations.
Robson Lowe in his Encyclopedia of British Empire Postage Stamps - Vol IV - The Empire in Australasia
, on p.505 lists two Plate Proofs of this design. One was in different colours again, and one described as "chocolate and steel blue on paper as 1d. (paper watermarked "WYNYARD BOND"). This stamp matches the latter description, except for the the watermark.
I welcome advice as to the status of this stamp.