The raised part is where the horn fits.
Yes, Robin is correct on the horn. With the enclosures there is no other easy place to fit the horn. On the rear casting of the ufm there are three through-bolts or studs. One for the motor mount, the next for the shock (coil over) and the third for the rear tub to hinge on. One the rfm the spacing of the cast lugs is closer.
I wonder how much of the stress problems for the F106 is due to the flexing of the ufm itself. It is certainly exacerbated by the lack of a rear cylinder. The C ufm is much stiffer. In fact, I believe the C ufm was a D Lightning part. All of the D Lightnings seem to have one. That says something.
I had considered making a Victor as I have all the fiberglass molds for the body work, a sub frame and a D tank. When I was at John Renwick's I saw his Folan with a D type ufm that he fabricated and I realized how easy it would be not make one using a stock steering head. John just made an 8" long steel box to replace the sheet metal oil tank and mounted the top tube in it with a bolt through the welded cross tube on the main tube. The rear lug of the ufm was simply machined out of steel to fit his needs. He had a threaded arrangement on the top tube so he could vary the rake to suit his purposes.
David