Use a locking cable release.

No.  The only way to keep the shutter open is to hold the shutter release
down.  If you do that with your finger, you'll introduce unwanted movement
into the photograph.  Thus you need to use a cable release, many of which
come with a simple locking mechanism, so you don't actually have to hold it
down for the duration of your exposure, you just have to remember to release
it at the end of exposure.

vibration.                    Bob Hickey

"bulb").  The difference is that a "T" setting requires that you push the
shutter release twice--once to open the shutter and again to close it.  The
"B" feature requires you to push the shutter release once--and the shutter
remains open until you release the button.

The Spotmatic F did not have the "T" option, it only had the "B" setting.
You would need to use a cable release with a locking mechanism on it.  You
would press the cable release button, lock it in place, then unlock it when
you wanted to close the shutter.

The Spotmatics were wonderful cameras, but this is one feature that they did
not have.

(From the original poster)

Hi All,

I had anticipated using a tripod when taking long exposures, so I wasn't
terribly concerned about a small amount of vibration.  I was also hoping to
find a way to avoid buying yet another piece of photography equipment.
(It's not so much the "buying" part that I dislike, it’s the negotiating
with my spouse to justify the purchase that I want to avoid.  Please don't
comment on this aspect as it could get to be a very long thread...)

Given the thoughtful answers posted to my question, it appears I have no
choice but to get a locking shutter release cable - possibly an
air-pressure-based cable.

I expect that the most difficult part of selecting a locking shutter
release cable for a Spotmatic SP F is finding the kind that would fit it.
Does anyone know the specific type of shutter release cable that would fit
a Spotmatic SP F shutter release button?

I hope I'm not stirring up any controversy on this next question.  My
intent here is to avoid getting a poor quality shutter release cable.  Does
anyone have any specific suggestions for or against a specific locking
cable?

Thanks again.

Don