The only difference between these two sight models is the outer diameter of the lock ring that comes in the sight kit.  PTR sight towers have a slightly smaller internal diameter than HK pattern sight towers do.  It's not much different, but just enough that if we made our sights fit the PTR sights, then they would be a bit loose on the HK sights and we wanted a perfect fit.  So we decided to make a different model for the PTR.  So if you buy an HK front sight and later want to put it in a PTR, all you would need is the PTR lock ring and it would then fit and vise versa.  The other parts in the kit are the same including the sight reticle.  So if you have both an HK pattern gun and a PTR gun and you want to get some different sight reticles, they would fit both sights.  Or if you wear out an o-ring, it is the same for both sights.  The only part that is gun-specific is the lock ring.  Since they look identical, there is a very small dimple machined into the side of the PTR lock rings to be able to determine which one it is.  

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