The two accessory catalog copies I have acquired from the internet list different options, and are not dated.
I believe that the speed increase for the Universal High Speed (UHS) head is 5:3, or 1.66 times the input. The UHS has 2 degrees of freedom and coupled with the cutterhead, gives 3 degrees of freedom. As Daryl says, coupled with the 10" horizontal indexer and the gearbox, spiral gears are possible, according to the accessory catalogs. The UHS also has an adjustable spindle, giving 2.5 inches of travel. There are different UHS models for the various mill model numbers.
There are two 10" indexers available for the VN mills, but one accessory catalog says only the horizontal one is good for all models, including the 12. The 10" Vertical index is good for all models Except the No.6, No.12, and No.16. When provided for a No. 6, 12, or 16, the spindle of the horizontal indexer is ground with VN "C" taper, same as the machine. Otherwise it is the VN #2 taper.
A second accessory catalog only discusses the NEW 10" Universal Spiral Index Center, with the VN "C" taper for the No.s 6 & 12, OR 112 collets for the No.s 22 & 32.
So, looks like a couple different versions out there.
The Universal Subhead mounts to the cutterhead and adds one additional degree of freedom, but has no speed increase.
The Gear Type High Speed attachment mounts to the cutterhead and gives a 3 to 1 speed increase.
There is also the Motor Driven High Speed Attachment used on the No.s 6, 12, 16 that replaces the normal cutterhead and has an electric motor to drive the spindle. Other catalog only lists 6 & 12.
Ron
P.S.
If someone has a dog plate for the 7 1/2" indexer, a dimensioned sketch would be nice to post, so those of us without one could make one.
Also, I could use some dimensions for the UHS spindle adjust handle and indicator, since mine is missing.