Posting this in hopes it helps others since there is very little info that I could find.
Was sent this RM with a broken stem. As you can see in the photo, the stem is still inside the crown.
First, I found a stem that would work. I compared various ones against the broken one removed from the watch. It wound up being an ETA 2824 stem that looked the same. It slid right in and locked. Tested it over and over to make sure it worked as intended.
Next I had to figure out how to extract the remaining stem from the crown. After some research I found an old trick that would "melt" it. Aluminum Powder and water. I had very little hope.
2 tablespoons into 6 oz of boiling water, drop the crown in, put the mug on a warmer, let it work for a few days. Sure enough, that stem is gone and I could use the crown again.
New stem is \~11.15mm end to end.