i just picked this up off e-bay, a Minolta Normal AF 50mm f/1.7 Autofocus Lens.
it also came with these Filters(straight from e-bay ad) all for $107 shipped

Olympus Skylight 1A
Prinz Split Focus +2
Pro C8 (or possibly CS - this is a minutely etched starlight filter)
Prinz PL (marked 504, this is a circular polarizing filter)
Pro 1A
Prinz 25A (this is a red filter, so clearly the original owner was interested in B&W)

my questions are what would one use a split focus lens for? and why
is the red filter used for B&W? i've never seen a split focus before.