In Europe, the rear rack manufacturers have decided on a standard so that light companies like Cat Eye can offer mounting solutions for cyclists using those racks.
In the US, there is no established standard for attaching light mounting brackets.
If you happen to have a 50mm rack you could use small part number 534-2250.
This is available by calling our Tech Support Center 800-5-cateye between 9-5 MST for $5 plus shipping and handling.
Lights compatible with 50mm rear carrier adapter 534-2250 include :
Lights that have a single bolt mounting point include:
-TL-LD1000 -TL-LD500
You do have some other options with your taillight. The light can also be
mounted on the seat stay (tube that runs from the rear hub up to the
seat post cluster). We hope that you can mount this on the left hand
side so that the light will be clearly visible to motorists behind. Small parts for mounting on a seat stay can be found in our small parts store.