Nice save. Probably half of the horizontal indexer headstocks that turn up are without the tailstock. Don't know why that is the case.
On the gears, as I'm sure that you noticed from the part numbers, most of the ones on the mill and all of the ones on the Indexer are Atlas 618 parts. I haven't actually bought any for the mill from Clausing but assuming that they probably have them, Clausing's price is probably competitive with Boston, without the need to machine it before you can use it. I recently bought the 44/52 combo for my 12" from Clausing and paid $30 something and $40 something.