It is a
SWAN 4500 Noodle Pen
You get beautiful thick & thins with just a slight adjustment in pressure...looks kinda like a brush marker but it has a thin metal tongue on the end to get the very fine hairlines & rubber instead of felt - I believe they're made in England.
Watch it beautifully demonstrated here by azavalia1
I WANT A SWAN 4500 NOODLE PEN FOR CHRISTMAS, or my birthday which is only 4 months away...should get here by then, right?!!!