I can explain, Jim. The 6 cylinder IMs all have these, as they pretty profoundly increase the strength of the frame for the back half of the car (if you look at a picture of a bare frame, you'll be able to see why). The massive increase in weight and power make this a sensible modification for the 6-cylinder cars.
When Scott Sloan had his 4-cyl car built, he had this incorporated into the build-- as the ethos of his car was "dual-purpose racer", and it was a pretty racy looking feature.
I had my coach built in 2005. I saw Scott's car that spring, and decided on the spot that I wanted to incorporate the same thing into my car, as I anticipated ending up with a very robust engine and transaxle, and I wanted the entire structi=ure to be as solid as a slab of granite (I had a couple of pan-based speedsters, and never could get used to watching to door-gap change as I drive over railroad tracks, etc.). I wanted everything to be "function over form", so "the brace" just added to what I wanted the car to be about. Scott's car and mine are the only 4-cylinder IMs I know of that have that brace.