At last, the mystery is solved! I'm extremely grateful to Beru-Chan for the following information.

Apparently when Rose of Versailles was first broadcast in Japan, (in 1979/80) by NHK and its regional relay stations, it didn't do as well as had been expected. One of the regional stations decided to give up on the series and end it early. They produced their own final episode to wrap up the series, and showed it in place of the original episode 24, cutting off the last 17 episodes. This final episode was called "Moetsukita Bara no Shouzou", "Portrait of a Burned Rose". It did not have the same production staff as the original series, so the character designs and art style are somewhat different (i.e., worse).

I'm extremely curious to see this alternate final episode, but apparently it was only broadcast once by one station and not released on video, though it was shown for a Rose of Versailles convention in Japan last year. (Ah, to have a whole con about RoV!)

Anyway, I'm so happy to finally know where my cel is from, and to know that it is Oscar and André, at least in some form, together. It's also kind of neat to have a cel from a "secret" episode. :-) One weird thing I've noticed is that while RoV cels from the "real" series are extremely prone to line fading, especially in Oscar's blonde hair, this cel is dirty but not particularly faded. If only they'd used the same paint for the cels from the "real" show!

Beru-Chan has a very pretty Rose of Versailles page here: