How do I determine what size stem I currently have?

I think its 120mm but thats measuring from the headset crown bolt to the middle of the handlebars.

I think I need a 70mm stem as I'm getting some numbness in my fingers when riding and if I reduce the distance from handlebars to saddle by 2" it will match the geometry of my old bike which fit me perfectly.

Measure horizontally i.e. level C-C (centre steeer to centre bars)