Top triggers as illustrated above have two problems: They're in the line of sight, and they produce a limited amount of leverage. For lightweight recreational bows, they are cheap and generally trouble-free in exchange for greater string-wear and a slight loss of efficiency as compared to locks that don't 'slap' the bolt.
As for historical documentation: Egon Harmuth's "Die Armbrust" has illustrations of top-trigger notch-locks for East Asia, and I have run into one illustration (very small) possibly in Harmuth... but I don't recall off the top of my head (which is why there's so little hair)... what was I saying? Oh yeah, I've seen a small period (Italian?) illustration of a crossbow that had no apparent trigger on the bottom, but a device on the top that might be either a catch or release-lever.
You will find the top-lever release clearly illustrated in the Osprey Men at Arms books, in their color plates. Trouble is, you can't really trust their color plates. When they're reproducing illustrations from the period under discussion, you're on solid ground, but when they get 'round to those lovely color plates in the back, you're at the mercy of their artists. They had a guy named Mc Bride, who could draw like nobody's business, but he always got the crossbows wrong in some important detail. So don't put too much credence in secondary or tertiary sources. Nuff sed. Geezer