They are probably original braces. Many times you will see the large members of the frame are hewn, and the scantlings, or anything smaller than a 5"x5", are sash sawn. A sash saw is usually water power, and is kind of like a pit saw and moves up and down. Sash sawn saw marks look a lot like band sawn timbers. The reason for this was that most sash mills were limited to the length they cut, and were most useful for sawing boards. It was much easier to hew a 9"x9" post than to have it sawn out. The last old barn left on my farm is a combination of hewn and sash sawn. The posts are less than 16' long, and are sawn. The plates are 40', and hand hewn, as there wouldn't have been a sawmill to cut that long. The braces and rafters are also sash sawn. Also the floor joists are 2"x8" planks that were sawn. It would have been hell to pit saw all those boards. In some barns that would have been built in an area without a sawmill, the rafters would be hewn, or made from whole logs, and the braces would have been riven from a straight, clear section of log. I've done this, and the clearer, the better! Riving (splitting with steel wedges and wooden "gluts", which were basically just wooden wedges
) would have been much faster than pit sawing.
To answer the question about the gap, that is a square rule frame, which means all the mortises were cut to the same size, probably to accommodate the largest piece of brace stock. It looks like they were cut for a much larger brace, and they ended up with smaller stock. Unless you want to custom fit each mortise, then you will have the same, although typically smaller, gap at the top of the mortise. If you had all of your brace stock cut, and it was all very accurately milled, say 1/16" variation, you could measure across at a 45 degree angle from the point of the brace, and get the length of mortise that you need.
Yes, Jay, French scribe is pretty amazing. One of my instructors worked on the new roof for the Cabildo in New Orleans after it burned. They hired a French timber framer named Frederic Brillant to oversee the project, as the Cabildo, as well as the Presbytere, were built by the French.