I have tried to look at all the pen reference sites, but cant seem to find an answer. All the ones on ebay seem to look exactly the same. Is it size?

Yes, it is size. xx2 is the smallest pen, xx4 the medium size and xx6 the biggest pen if there had been a xx6 at all.

There has been a 256
But unfortunately 99 of 100 256s have cracked caps by now. because of a too strong tension and a weak plastic. The polymer plasticizer of these early pens from the 50' are more or less gone and so the plastic tends to break very easy.
To store the 25x pens one should not push the cap on the pen too hard.

Axel

Are there problems with the 254 too?
I have a nice 252 but it is a bit small and I was looking for a 254 but could not get myself to spend the money.
Probably because I cannot make a decision if I like to go for the 254 or another 14 .....

Granted, I may lead a charmed life, but none of my 25x have cracked caps, nor does my 14 have a cracked section.

Having said that, the 254 is closer to the 252 than it is to the 256 (this is all subjective). That is, I recall the girth stepping up more from 254 to 256 than 252 to 254.

As I recall, the 254 is very close in feel to a Parker 51, that well known universal size comparison pen.

There's something about the wing nib that is very appealing to me. I like my 14 to use, but like the 254 to use and look at.

gary

Well I clearly have a lot to learn about Vintage MBs...
I like bigger pens, so I suppose I am after a 256 then.