MP Brian Walker
I found, like you said, a bits of here and a bits of there. Seems like he leaves a lot of bits everywhere. Most interesting part is the Spencerian Study Group, IG ink, and tips about getting it right with the script. There is no such group left except for IAMPETH. Personally I like the company found here, its much more a group of fine pen(wo)men helping each other.
About the Leonardt Principal, who is actually making them now? Manuscript?
Here's my very first attempt at nib reviews. I much prefer the Esterbrook 357, Heintze 2255R, and the Hunt 101 USA to all the nibs. The Leonardt Prinicple did not jive with me, too stiff, less pointy, and more scratch. The two best for hairline and glides smooth and easy to flex is the (first) 2255R and (second) Esterbrook 357. Please understand this is my first attempt at dip pens.
The ink have a very important roll, I use Parker Quink with 1 tea spoon of raw honey and 10 drops of PhotoFinish. Why did I do this additive combination? PhotoFinish lowers the ink surface tension, the honey makes it more elastic.
There are lots of flexible nibs (vintage) to be found. This is just my sample from a Deutsche calligraphy site.