When to use downthrust jaws? When NOT to use them?

Hi Group,

I just bought a used pair of downthrust jaws for my old-but-good Hilma 125mm hydraulic vise (from the mid-1960s).



The jaws are model 3722-03 in this catalog. They work very well: if I put a workpiece on parallels, then clamp, I can't pull the parallels out from underneath. With normal jaws, I could slip out the parallels from underneath.



Here are my questions:
- Should these be used as pictured above, with the ordinary hard jaws in place? Or without them?
- What is the purpose of the long slot running along the bottom of the moving part of the jaw?
- When should I use these jaws, and when should I avoid them?

Thanks for reading!

Cheers,
Bruce

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