Looks like I found the "resident expert" on Winchester O/U shotguns, as recommended on another forum:
"I believe the 99 (as it was called in Europe) is the same mechanically as the Winchester model 96 in US. It would appear so from your pics. If so, I have the right ejector, new, priced at $65 plus mailing. The ejector spring appears to be there, but I don't see the spring PLUNGER, which is additional $12. You may also need the ejector retainer (shaped like a small donut, see opposite side for reference). This is also $12."