I wondered the same thing. Like you, I have not seen another Rock River with front strap dimensions like this one. I bought it essentially new-in-box a few months ago. From what I understand, this pistol was made and assembled by RRA (not sold in the white) circa 2000/2001. It may have been made with a lower cut at the request of the original owner. Perhaps others on the forum more knowledgeable than I can comment.
What I do know is that this gun functions flawlessly, has a stellar (DCM legal) trigger, and is very accurate. I really like the classic lines and the clean slide. I would love to find a Limited Match to go with it.