The Dillon 550 and 750 Primer Stoppers work in the same way due to design changes, the primer bar on the 750 is longer than the 550. Some people may find that the 550 stopper works in their 750 however the 750’s stopper will not fit the 550.
First, pull the handle of your press to the downward-most position and stop. With the primer bar in it’s full rearward position, insert the stopper with the forked-end facing towards the press. Return the handle to it’s normal position and check for function.
Some may wish to release the return spring underneath the primer bar and remove the operating rod from the press so that it does not flap around while the press is in use.