China Republic stamps : 04 Dr Sun Yat-sen 4

The second London printing of the Sun Yat-sen stamps that were issued between 1931 and 1937 have a special peculiarity.
Prior to 1933 a wet-paper printing proces was used. This resulted in shrinkage of the paper and therefore the image is 1 to 1.5 mm narrower than the later dry-printing issues.

above : dry-printing process : broad image
below : wet-paper printing : narrow image

In general, the average width of the image is 19 mm. Smaller images are to be considered as 'narrow', wider as 'broad'. In most cases, the narrow image is higher valuated than the broad one.

2 cents

For most collectors, this difference is the deepest level to sort out their stamps.
But why stop, if you can go deeper...
According the chinese catalog, there are 3 subtypes of the 2 cent stamp.

subtype A

has a retouched outer circle
 

 
the shoulder has an incomplete line
underneath the collar

subtype B

the outer circle is also retouched

the shoulder has no incomplete line
underneath the collar no more

subtype C

the outer circle is now complete

there is no broken line
underneath thecollar

Other differences can be found in the YU-chararcter (5th character from the right).
郵 (yóu or yu)

郵 The 'waffel' shape is in the A-type more like squares - in type B and C the right border looks like two hooks. The first image is a type A - as it is the 'narrow' version, which only has the A type.

In the 'narrow' version, only types A are known (early printings).
In the 'broad' version (later printings) all 3 types occur (type A is more rare)

to be continued ...