I don't know if you want to keep it true to the original, but I think the 3, 4, 5, 6, 7, 9 and X are too wide. Y too narrow. Some characters are too heavy, like M, S, T and U. B is probably too light, as is C.

Oh I heartily agree they are imperfect. The numbers in general are much too heavy, and the N and X didn't even exist in the original. It is most assuredly time fore some cleanup, but I'm doing this mostly for the experience of translating hand-lettering to digital type, and there are flaws inherent in that process.

I swear I've seen a similar commercial font somewhere, but I just can't place it in my mind. Hmm...

Yeah, it does look somewhat familiar.

Reminds me of some of René Knip's work. If that's what you're thinking; then there's no commercial font; I don't believe he has released a retail design yet.

Yeah, it does resemble Knip's style a bit (which isn't a problem). :)

If you finish this, I'd make one version with the arrows retained.

Well, to include the arrows I'd have to *gasp* learn how to draw these letters properly, since I'll have to make them up for all the characters not in the original sample.
I don't think I even have a Speedball A-1 nib.

Well, there are probably not many ways to draw the letters you added yourself.
Looking at the image quickly shows that there are only horizontal strokes from left to right and downstrokes.