this is what I have been taught, during vulcanizing, if the lid touches the 1 inch frame, then the mold may not vulcanize properly. meaning it didn't get enough pressure to squeeze the rubber to fill in all of the detail leaving a bad spot in a cavity.
My frame is taller and I put in a 1/2" spacer disk on top of the mold, so I can may a 1-1/2" thick mold if necessary.
I went and checked the lip on the bottom and found out I measured the bottom disk when I told you 1/4 inch
the bottom disk has a 1/4 in lip and the top has 1/2 inch.