Never seen any stone body look remotely like that. I have both an old non-Platinum Onyx Signature, a low serial-number Onyx Platinum, Coralstone Platinum, and Jade Platinum, lol. Platinum serial numbers start with a "P". Older Koetsu builds don’t have threaded mounting holes, and older still will use gold pins instead of silver. Older still, and you get silver top plates (like my Onyx Signature). And long-bodies go even further back than all of those. The top plate, mounting holes, and pins are not changed out by Koetsu on a rebuild (I know because I asked for it and they wouldn’t change it).
I think (but cannot verify) that any genuine stone body with threaded holes and silver pins would be recent enough that it would have to be a Platinum model (all stones became Platinum at some point), and should have a "P" serial number on the top plate. Your pictured example violates this, and furthermore it features the simplified gold cover/guard plate without a raised "lip" -- which is a feature exclusive to the modern entry-level Black and Red (Rosewood Standard) models.
So I think it has to be considered a strong possibility that someone shoved the guts from a modern Koetsu Black/Red into an Onyx shell (which itself may or may not have been genuine Koetsu). Either way, sadly there is about zero chance that this example actually has Platinum magnets. It might still sound very good, if the guts were genuine Koetsu and have not been unduly worn or tampered with. But if you were sold a Platinum, I think you did not get that :(
I think (but cannot verify) that any genuine stone body with threaded holes and silver pins would be recent enough that it would have to be a Platinum model (all stones became Platinum at some point), and should have a "P" serial number on the top plate. Your pictured example violates this, and furthermore it features the simplified gold cover/guard plate without a raised "lip" -- which is a feature exclusive to the modern entry-level Black and Red (Rosewood Standard) models.
So I think it has to be considered a strong possibility that someone shoved the guts from a modern Koetsu Black/Red into an Onyx shell (which itself may or may not have been genuine Koetsu). Either way, sadly there is about zero chance that this example actually has Platinum magnets. It might still sound very good, if the guts were genuine Koetsu and have not been unduly worn or tampered with. But if you were sold a Platinum, I think you did not get that :(