[Crosspost from WAYLT2]
Meanwhile, this afternoon an operatic rarity.

The above is not the issue I'm listening to, but is the only Amazon listing that has an image with singers and performance details. The one I am listening to is an Opera d'Oro release (with reasonably good sonics) that is listed here with no image.
Since it's Opera d'Oro, there's no libretto. Reading the plot synopsis on Wikipedia, that may be a good thing. The plot is wildly unhistorical (Philip Augustus gets himself killed in a duel while pretending to be the Duke of Burgundy on an embassy to the Holy Roman Emperor) but the music is well handled. There are no bel canto fireworks (the opera was composed for the 1830s Prussian court) but a good deal of melodies and ensemble singing. Think Rienzi (Wagner admired Spontini's operas, so it's probably no coincidence.) By necessity the performance is heavily cut; the original supposedly ran over four hours.
As an alternate to this performance, the only option is a recording from the 1954 Maggio Musicale Fiorentino under Vittorio Gui with Corelli and Udovick, of which several versions seem to be available. This 1970 performance seems to be harder to find. The one to which the image belongs is labelled "currently unavailable".
I remember seeing a listing for a Myto issue but can't locate it now, and there don't seem to be any other options.