Thanks Dave?
The base casting of the Packard is very different from the std RF or HF.??
When you think about it, the whole vice table is stronger without the curved slot and the angling of the 'fixed' jaw easier.
Probably cheaper to do it with curved slot and smaller fixed jaw, but not as rigid or as accurate as the Packard. because the std design often warps across the vice table from the reduction in metal.? This means the work doesn't stay parallel to the pivot and so cuts out-of-square.
Here's a design feature for the redesigned RF - jv
John Vreede