sti block
 

how do you know a sti block from a wrx / uk 99 on . is there any external numbers or marks ...

They are one and the same thing.

David APi Engines / APi Impreza

PS. although some clever clogs will post on here about the closed deck block being special. They are stronger, but only fit the centre thrust crank unless you machine it to be different and take the later crank. They aren't especially STi just stronger.