I'm deep into the early history of steel pens at the moment, with a focus on 1820-1860. This was the period when steel pens went from a luxury novelty to an everyday staple, driven almost completely by the British pen manufacturers.
I'm also starting to get an answer to the question I've wondered for a long time, when do we first see oblique holders?
Well, I finally have an answer. First you have to go back to this period I'm looking at in which you find the first real pen holders. Prior to this period, steel pens were almost universally all barrel pens affixed to a holder pretty permanently. Pens were not really disposable. There were even steel pen repair services, just like the same services to repair your fine quills.
Individual slip nib pens which fit into a holder were originally pieces of a quill which came in a box of nibs and fit into a holder. These were disposable and meant to obviate the need to mend your quills.
But the oblique holder seems to come with the first oblique nib. By 1831 you did start to see more slip nib pens, but the idea of holding the nib at an oblique angle in the holder seemed to be a new idea.
In 1831, an enterprising and very successful stationer and inventor, Sampson Mordan combined with one William Brockedon to patent the first oblique pen and oblique holder. (I've attached the patent below)
In the patent application they mention the benefits as this will allow the writer to hold the pen more comfortably as well as it should allow the pen to last longer since both tines will be moving across the paper evenly. If you read the description, the idea of holding a pen obliquely seems to be a new idea, and one that requires explanation and justification, and the obliquity itself is patentable.
"and we hereby claim as our invention, the oblique direction or position purposely given to the slits of all pens, whether made of quills, metals, or other fit and proper materials, and also the obliquity produced in the use of common pens, whether made of quills, metals, or other fit and proper materials, when held in our oblique pen holders."
The holders shown in the patent document include oblique nibs, as well as oblique holders with straight nibs. Figure 17 is explicitly labeled as "another pen holder adapted for holding common quill or metal portable pens [slip nib pens] in an oblique position"
Figures 26 and 27 show the oblique nib which was also covered under this patent.
The explicit language of this patent make it clear that the idea of holding a pen at an oblique angle is the central new idea of the patent. And thus with this we can finally point to the beginnings of the oblique holder and oblique pen.
One mystery solved, 10,473 more to go.

P.S.
Thanks go out to the owner of the Sampson Mordan site
http://www.sampsonmordan.com/. I have found the various "British patents related to steel pens, but have not been able to access any details. This wonderful person has posted some of Mordan's patents and fortunately this was one of them. It helped me confirm what I suspected about oblique holders.
If you want to search more British patents, at least the names and patent numbers, I have the only list of indices I've found for the early patents (pre-1881) on my blog.
https://thesteelpen.com/ I had to gather them together and it's still incomplete, and there are no details like the attached.
If anyone has access to these pdf's and would be willing to grab a few, I would be most appreciative. Thanks!