This appears to have been the mid-range 'workhorse' for the whole of the latter half of the Curzon-Summit production life.  There appears to have been two distinct types of 125:

Summit S.125 Mark 1

Measurering 132mm approx, this model comes with Gold Plated Trim. Most carry the Summit name on the clip, but below is a NOS item with a plain clip. I do have examples that have both Summit & Warranted 14ct nibs.

five S125 examples

There are clearly two distinct shades of Green marble. The Black pen has a chased appearance.

125 mk1 bs

Summit Belgor

This model also comes in a version that carries the Summit-Belgor name. There does not appear to be any other differences between the two pens. Also found on the internet was this example,  described as a "S.125 Summit-Belgor". 

125 belgor

and from my collection a green example;

125 belgor 


Summit S.125 Mark 2

This later model has a "cheap" feel to it, which may not actually be justified as the examples I have come across have all stood the test of time extremely well. At 130mm it is slightly smaller than the earlier model,but it is the lighter weight and ribbed appearance that are more noticable;

s125 later

The Mark 2 comes with GPT and a Summit 14ct nib as standard. I have yet to find a green or burgundy example, but assume they were produced. The blue "Stockwell" example above indicates that this model was used for the 'own brand' market.

125 barrel stamp

The barrel stamp curves around the pen immediately below the screw thread.