Does anyone have a record of a rated game in which an opposing king was mated

by king bishop and knight within the required move number? (Fifty, I think)

No, I don't  But I thought it was 30 moves or something?

I think it's 35 moves with ideal placement for Black.  As far as "rated games" go, it is not unusual that a player will make his opponent demonstrate he can do it in open tournaments, at least in games between non-masters.  The ending itself is a rare occurence, so the number won't be too large, but it happens.

The late IM Rich Delaune told the story of his first rated game in the early '70s, facing an A (1800-1999) player.  He was tearing the guy up on the White side of a Sicilian, and had blown open the f-file.  Rich:  "So I was thinking to myself, 'Gee, this might be my most brilliant game ever!  I've made several brilliant moves!'  Inspired by my own brilliance, I reached out and played the equally brilliant Rf8+, which only failed to KxR, which I had overlooked."

Fortunately, Rich was still winning, but the game - his first rated tournament game, remember - boiled down to B+N v lone K, and his opponent didn't resign.  Rich mated him.

 Lol. If that were me, I would probably be staring at the board for a full half an hour per move and draw on time.