Although this type of nib is often referred to and sold as a mapping pen, it's also called a crow quill as distinct from non-tubular mapping nibs like the Hunt 103. As the name suggests, their primary intended use is for drawing, including cartography. They're the finest of the fine, suitable for the tiniest of details, and generally speaking very high quality.
I haven't used the 801, but have played about with quite a few vintage crow quills as well as the three Gillott varieties (2788, 850 and 659). They're extraordinarily challenging to use, and very smooth paper is a must - for me at least. Ideal for small pointed pen scripts provided you have a very deft touch - that's definitely not me yet. Almost anything else would be easier to get started with.
Oblique holders to take them are not so common, by the way: the Paper and Ink Arts adjustable is the only off the shelf option that comes to mind.