Can anyone tell me some physical dimensions of the 600 nib? I have a couple of 400 nibs, and a 600 nib. I know that it might seem obvious to tell them apart. But, it is not. In case there is any vagueness, all of my pens are of the new type.

Any tips?

You could always look at the table in nibs.com here: look at table somehwere in the middle

Wow. According to that chart, the 400 nib is actually 1/32 inch LARGER (or shall I say LONGER) than the 600 nib...

1.5" (marked 38+mm ?) for the 600, 1 17/32" (38 mm) for the 400.

Is there a GIRTH measurement? Some difference in metals, or scrollwork on the nib itself?

I think the feeds looks different. The first fin (next to the metal collar) is thicker than the others on the M600 where they all look the same size on the M400 IIRC. I remember you need to unscrew it a bit to check. But the 600 nib is also a little longer.

--Doug

Just to share the answer I received backchannel, and for posterity:

The 600 feed has FOUR fins below the collar (inside the barrel), while the 400 has FIVE below the collar.

I will be more vigilant in the future...

Haven't found one yet for girth. The nib difference, you already answered

Below is a compatibility chart from Richard Binders site. I didn't see any measurements for the nibs but a photographic comparison does show a difference between a M600 and a M400 nib.

This nib… …will fit into these pens.
M1xx[2] M1xx, M2xx, M4xx, M6xx, M7xx Toledo, “1931” and “1935” LEs
M2xx[2] M2xx, M4xx, M6xx, M7xx Toledo, “1931” and “1935” LEs
M3xx M3xx
M4xx[2] M2xx, M4xx, M6xx, M7xx Toledo, “1931” and “1935” LEs
M6xx[3] M6xx
M7xx Toledo[2] M2xx, M4xx, M6xx, M7xx Toledo, “1931” and “1935” LEs
M8xx M8xx, M9xx Toledo
M9xx Toledo M8xx, M9xx Toledo
M10xx M10xx
“1931” and “1935” LEs[2] M2xx, M4xx, M6xx, M7xx Toledo, “1931” and “1935” LEs