What's this threaded extrusion on my Sinar back?

[Sibben]

On my revolving Sinar Norma back there's this mysterious extruding knob with threading inside. What am I supposed to fasten here?

109651

[Leonard Robertson]

If you have the Sinar behind the lens shutter, there is a special Sinar cable to connect the shutter to the threaded hole on the back. Then when you insert a sheet film holder, the cable closes the shutter (which was open for focusing) before you have a chance to pull out the darkslide and ruin a sheet of film. I don't have this cable for my Sinar shutter. Seems like it would be a handy thing. I've read other folks comment that it works well in the studio, but for use in the field it is just another thing to take along and hook up.

Len

[Sibben]

Cool. Thanks.

[Steven Tribe]

You have a 4x5 Sinar.

The other sizes have a less obvious female thread.
I enclose a photo of all three sizes, designed to fit on an 8x10, but they look the same on backs just designed for dedicated rear standards. Increasing size from left to right.