That green splitter you posted the advert for is pretty pointless from a safety perspective.
You're best off thinking of the black plate as a support for the crown guard, as it can't function as a riving knife.
The idea (of a riving knife) is that it prevents the wood either pinching or otherwise moving into the path of the rising teeth at the back of the blade. If the wood doesn't do that, and if the top of the blade leaves the wood (i.e. it's a through cut rather than a groove you're making), a kickback is unlikely. The riving knife needs to be only slightly thinner than the saw's kerf, so that it passes through the kerf as it is cut, but stops the wood from deviating right or left. The general rule of thumb seems to be very slightly thicker than the saw plate (the main disc of the saw to which the teeth are usually brazed). So if you change the blade thickness you really should change the riving knife, too!
It looks horribly as if that saw has never actually had a riving knife fitted at all. As discussed, one would expect it to rise and fall with the blade, so typically it would be mounted to something associated with the mounting for the blade arbour (it's spindle), and would tilt with the blade, staying close to the teeth and in alignment with the blade.
As it stands, the black steel plate ought to be quite a good support for the guard,
when the saw is used to cut squarely, meaning 90 degrees from the table surface. But it's not a riving knife, and if you tilt the blade at all you'll have to remove both the plate and the guard, which is wholly unsatisfactory.
It isn't a riving knife and it can't ever be one as it's far too far back from the blade and can't be made to stay close to it.
As things are now, they don't make commercial sense (the saw was originally sold by a UK firm, after all). Riving knives have effectively been mandatory for new UK saws for a long time.
I'd strongly suggest you do a little detective work to see if the mechanism around the arbour shows any signs of missing parts or bolt holes where something ought to be mounted, but isn't. Contact Poolewood too,if you can, as they may well know exactly what ought to be there, and might even have the missing parts.
Personally I wouldn't feel comfortable using it in that condition, although I appreciate that thousands of Americans do that sort of thing every day. But they also allow individuals to own assault rifles, so it's probably sensible not to extrapolate from them...
E.
PS: you even find riving knives on handheld circular saws. The Festool TS55 has one, but my Makita rail saw doesn't. Guess what:
I actually managed to get a kickback when using the Makita on its rail.. The kickback caused the saw to jump off the rail and was extremely frightening, although no damage was done: if you lift a plunge saw off the rail, the baseplate and the blade cover immediately spring down to protect you, but if the saw comes off the rail at an angle, the spinning blade is exposed, and probably pointing at you (because of the way they're usually used). A stiff, sweet coffee was called for, and an important lesson was learned - you'd think there shouldn't be enough kinetic energy available to do that, but there is (and your finger is still on the trigger so there's power from the motor, too). I really like the Makita in every other respect, but, bluntly, it should have a riving knife.
