I have two 9556 nibs and they both have different styles ofimprinting on the nibs. For example, on one nib, the 9556 goes horizontally across the nib, on the other it runs veritcally. How many times did they change the style of the same numbered nib and could I tell the approximate year of manufacture from the design?

Well, Paul Hoban's book shows only horizontal lines on all 9XXX nibs which is a copy of the Renew Point selection chart so I guess the search is still on.

A (very) wild guess might be that the vertical imprints are later chronologically, but
I have absolutely no proof for this. Maybe Brian knows something.
Relevant or not, all ads that I have seen show a horizontal imprint.
The later nibs with Fine/Medium/Broad designation are vertical.

I didn't realize this nib might be uncommon. Here's a photo to help shed some light on it.

My 9128 is that way, and my 9550 is the other way. I think it probably just varied by the period it was made it.

I don't think it's uncommon. My 9550 and 9556 imprints are vertical, same goes for one of my 9128's - but then I also have a 9128 where the imprint runs horizontally.

Photo here: http://www.fountainpennetwork.com/forum/in...st&p=279960

The Renew Point chart I mentioned above matches Phthalo's pen on the right in her photo. Now I am really intrigued.