@Pete – It seems I wasn't clear enough. Like I say, the problem with all plier that have freely hinged jaw faces is that it is almost impossible to apply the same pressure to both end of the jaws at the same time. Indeed the ONLY way that you could achieve this would be to insert a part EXACTLY between the two hinges.
i.e. You have to locate a part at Position A.

For a small part this is horrible, because it is surrounded by over-hanging jaws.
i.e. If you locate a small part at Position B, the jaws just pivot and are no longer parallel.
The only possible option to hold a small part at the end of the jaws (e.g. imaging a small 1x1x1cm cube) where you can easily work on them, would be to insert and part of identical with at the opposite end (i.e. Position C).
@Clive – a parallelogram should be able to move things in parallel. The direction of movement depends on the chosen direction of the parallogram.
To my mind it is only a "bodger's tool" because of the teeth that will tend to mangle/bite into the part you are gripping. I don't care about weight.
@DK31 – Yes, but the "140 MM SIDE SCREW PARALLEL ACTION FLAT NOSE NYLON JAWS PLIERS JEWELRY EXTRA JAWS" are the same broad design as my original "Option A." i.e. Yes they have a nice parallel action, but they can't be made to grip like a vice.
On reflection, re my post at 05/05/2021 23:57:10, I see now that the core problem with the STAHLWILLE 65622250 (and it's cousins) is that one jaw is static. This means that as you open the jaws really wide that the jaw that moves will be going through some sort of arc (i.e. back towards the users hand).
* * *
OK to put my quest another way, I would like to have "a hand tool that is a fine-nosed vice – with a true parallel jaw action, and no irritating protrusion near the business end".
e.g. This attempt at a hand-held vice ("Precision Hand Vice Work Holder"
tried hard to have parallel jaws (like a vice) but that sticking out head of the adjustment screw gets in the way of access pretty badly.

Probably not worth much more time thinking about this thread. Maybe what I seek hasn't been invented yet?
Edited By John Smith 47 on 06/05/2021 10:51:32