I already had this 1969 stamp from South Africa, but these control blocks were for sale for just a few dollars, and I bought them.
The two control blocks were printed on different types of postage stamp paper.
(Are these two distinctly different postage stamps— and should the philatelist put one of each in his or her album? I say yes: if you have both types, put both in your album.)
When the stamps are held up against a light, it’s easy to see the watermark clearly, in the white margins of the block.
These are some of the last South African stamps issued with watermarks.
In the United States, stamps with watermarks were issued only for a short time— from 1895 to 1916.
Issued Oct 6, 1969
Perf. 13½x14 | Photogravure printing on Harrison paper | Watermark RSA in triangle
357 A140 2½c Ocher, Prussian blue & yellow
Harrison paper has a grey or fluorescent back.
Issued Oct 6, 1969
Perf. 13½x14 | Photogravure printing on Swiss paper | Watermark RSA in triangle
357 A140 2½c Ocher, Prussian blue & yellow
Swiss paper has a dull pink non-fluorescent back.
It’s hard to tell from these scanned images, but comparing the physical paper with the Harrison paper, the Swiss paper seems to look just a touch whiter.
*Upright and upside down next to each other