It would be a #2 size, a universal #1 was never made, so far as I know, so it should be approximately 9" swing. The geared feature of the universal is not just about cutting spirals, but also it has the capability of differential indexing; the capability to divide prime numbers. In that mode, the spindle is geared back to the dividing plate, so that it moves while you are cranking the worm gear handle in order t divide a number that otherwise would not be possible without a dividing plate with the number of divisions desired; in the case of a B&S head, some numbers, such as 127 (for metric conversion) is not possible due to the small diameter of a plate not accommodating so many holes. The Cincinnati type div. head makes this possible by plates that are much larger diameter, and there are prime number (special) plates available.
I have a universal head, and use it often on my mill, both for plain indexing, spiral cutting, and differential indexing. It is of B&S manufacture, and is considerably nicer and more sophisticated than the Asian copies. One nice feature is that the sector arms are graduated, and in the table of divisions, there is a column titled "graduation"; set the arms to that number and there is no need to count holes in the plate, setting to the number automatically sets the proper number, avoiding the possibility of a mis-divided (scrap) part.
