Seiko Shutter Mystery Lever

[Seasider]

Forgive this newbie question but I am new to large format photography, so I'm unfamiliar with some of its components. I have, for a 4x5 camera, a Fujinon W 6.3 lens in a Seiko shutter. I'm clear about setting the aperture and shutter speeds (B up to 1/500), and cocking and firing the shutter, but I'm foxed by a lever that lives in a small slot to the left of the red dart on the speed selector ring when the shutter is set to B. It moves a little to the left, then returns when released. It doesn't seem to do anything, so I have no idea of its purpose. Can anyone help? With thanks in advance.

[sharktooth]

It's almost certainly a lever to manually open the shutter blades so that you can focus on the ground glass. It's usually a two position lever or slider tab, depending on the manufacturer's preference. Just make sure you close the shutter blades with this lever BEFORE you remove the dark slide on your film holder. We've all been there, done that.

[Alan Klein]

It's almost certainly a lever to manually open the shutter blades so that you can focus on the ground glass. It's usually a two position lever or slider tab, depending on the manufacturer's preference. Just make sure you close the shutter blades with this lever BEFORE you remove the dark slide on your film holder. We've all been there, done that.

The OP says it returns automatically to its original position. The manual shutter blade switch doesn't normally return automatically. The shutter release does that. Can the poster show a photo of this lens and which switch he's referring to??

[gnd2]

Based on some google pics, I'm going to say cocking lever for the flash sync timer.

[Alan9940]

There is a lever to open the aperture blades for composing/focusing, but the shutter needs to be cocked or the lever does nothing. What I do is 1) cock the shutter, 2) flip the lever to open the aperture blades, 3) do my thing, 4) close the aperture blades, 5) fire the shutter, then 6) set needed shutter speed and aperture and take picture. Hope this makes sense.

[Seasider]

There is a lever to open the aperture blades for composing/focusing, but the shutter needs to be cocked or the lever does nothing. What I do is 1) cock the shutter, 2) flip the lever to open the aperture blades, 3) do my thing, 4) close the aperture blades, 5) fire the shutter, then 6) set needed shutter speed and aperture and take picture. Hope this makes sense.

Many thanks to all responders and particularly to Alan. The mystery lever does allow one to preview any aperture setting but it only works after cocking the shutter - that's the bit I was missing out.