If you are unsure what size rod butt you currently have use these measurements to ensure you order the correct boom size.

The Ledge Logic #6 (130lb class) Boom is made to fit into a rod butt whose opening is 1.336" 

The Ledge Logic #4 (80lb class) Boom is made to fit into a rod butt whose opening is 1.133"

These are very specific measurements and it may be difficult to achieve a reliable result with a ruler.