The "hold down" is the part of the vacuum advance unit with the slotted hole called the "octane selector". DO NOT install an O ring in that top groove- - - -it interferes with the free movement distributor housing by the vacuum advance. That groove is there to keep oil from migrating up the housing and it does its job by simply being there- - - - -no seal needed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!