In answering your original question, " What's the screw mount for in the helical " ?
I just found this out, because I saw one for sale on e-bay, I've had these close up rings and didn't know what this particular one was or how to use it.
Basically there seems to be two types, one type mounts like a regular lens in the helical, then the lens mounts as usual into the close up ring.
The other type, I wasn't familiar with, screws into the helical then you turn the lens around and screw the lens onto the close up tube using the threads that would usually be used for a filter.
When I saw one listed and described on e-bay, I tried it and, Volia!!, close up photography!!
Hope that helps and enjoy your S2a..
j