I have a feeling this is a part that is often broken on these old double guns. Wisner's http://www.wisnersinc.com/ has them for $25 plus $7 shipping. Their catalogue says it is for the extractor model, but they will fit the ejector guns as well.
It was a smidge too wide and I had to dress it down with a file, but it works perfectly. I find it somewhat surprising that a company is still making parts for a shotgun that hasn't been produced since 1910.