If you have heaps of rulers lying around and want to save $$ and buy only the vernier head then this is for you. These heads will fit most high quality rulers as far as we are aware, such as Toledo or others.
Made to fit rulers made under the JIS manufacturing standards which are common sense sizes:
300mm ruler is 25 x 1.0mm.
600mm ruler is 30 x 1.2mm.
1000mm ruler is 35 x 1.5mm.
These precision made Vernier Scale Stoppers most likely won't fit the generic Big Box Store version of what they call a ruler. If you are unsure - for another $18 you can get the full kit with brand new rule included from related products below.
For example 25.4mm x 0.87mm ruler size will not work with the 300mm stopper. It must be 25.0 x 1.0mm section size.
We also recommend using a fine honing stone to run over the edges and corners of your older rulers so the new Vernier Scale Stopper head slides nicely without tearing out its insides on even little dings and nicks that plague an old workshop ruler.