this is a new replica petrol fuel tank cap assembly to fit all  the above european-specification XS500/650/750/850 models. 

Fits   XS250  XS360  XS400   76-78   type 1U4 / 1U5    76-78

         XS500 C D E  76-79  type  1A8 / 2F1

         XS650  C  D  E  F  76-81

         XS750  D 2D E F  77-78  type 1J7 /  1T4 / 1T5 

Generally,  it does NOT fit  650/750/850  SE/Special/Custom/USA/Heritage  specification  models.  

Does NOT fit  XS250/400 type 3N6  ,  or XS250SE/XS400SE type 3F9/4G5

It is approximately 115mm long

If in any doubt about fitment please ask,  ...tell me your model details/frame number etc.