In John Easton's "Postage Stamps in the Making" published in 1949, the following appear under a section on Paper problems and remedies:
Irregularities in the felts between which hand-made paper is pressed, or the cylinders coverd wth felt in the case of machine-made papers, frequently gives the impression that a paper is ribbed.
The so called ribbed paper on some early Austrian issues were caused by the newness of the felt on the cylinders.
Something for you to investigate.
Wilding Mad -
The type of stamp paper that I have found sounds similar to the case of the Austrian variants as you have described, I have checked these reversed (single ribbed) papers under long wave UV to find that some are on a cream type of paper whereby others are of the whiter variety, therefore the paper must have been produced sometime around 1962 during the change-over period.
There is zero correlation between the Austrian stamps referenced and the Wildings which are the subject of this topic.
In the case of the Austrian stamps the paper has a ribbed-like texture that can be felt and seen with the naked eye. It fits the classic definition of ribbed paper.
The Wildings do not have that feature.
We keep coming back to calling the Wilding papers something which they are not, ribbed paper. They exhibit a watermark-like feature when in fluid much like many stamp papers do.
Let us put this to rest.If you measure the Wilding paper with a micrometer at regular intervals such as where a line appears when the stamp is dipped and where a line is not visible do you get different measurements?