My SCSW 3rd edition defines the flat latch as "Thumb latch configuration used on I and J frames and Airweight K frames from about 1950 to 1966." It gives photos of 4 different latch styles, newest to oldest:
1. Current - appears to have a piece cut off from the 3 to 6 o'clock position, probably designed for easier use when pushing it forward from that location. Large screw towards forward end.
2. Traditional - More of a square appearance I believe you're referencing. Large screw towards forward end.
3. Flared flat latch - flat and rectangular shaped. Small screw in middle.
4. Early flat latch - slightly humped and rectangular shaped. Small screw in middle.
What makes one more valuable/desirable to me over another is whether or not it is original to the gun. I love the unique appearance of the flat latch but it would sure look out of place on a newer gun.
Hope that helps!
Todd