Rod ends sold in sets with hollow balls.
M3 ends are made for 1/8" rod ends and or shocks, they will come with 6.3 steel rod end balls
M4 rod ends are made for 3/16" Links and will come with 7.0 steel rod end balls
M4 rod ends are a direct replacement for Traxxas TRA5347
M3 ends are a direct replacement for Traxxas TRA1942 hex
M3 short are a direct replacement for Traxxas TRA2742X