That spacer is not standard on a T140 1977, it may have been added to beef up the springs a little.
standard fork tube length is 555mm
standard spring length 485mm
The top nut should be in two parts, top hex nut in steel with the other part made out of alloy.
If you remove the top nut you will find it has an allen key hex inside, normally the chrome instrument bracket is mounted between this two nuts. (hope that makes sense).