Mike101 - Lett grips did indeed make the factory OEM grips for Ruger. However, they have gone out of business and the web site, although accessible, is not supported any more.
Here's the skinny on ROA and Blackhawk grip frames as they relate to fitting grips: they are completely interchangeable. ROA grips will fit on either an Old Model or New Model Blackhawk and vice versa.
However, the frames themselves are not interchangeable. The NM Blackhawk uses a different trigger and trigger spring than the OM Blackhawk and ROA, and the grip frame is different to support that new design. An OM Blackhawk and ROA use the same trigger spring mechanism and thus the grip frames will interchange between them.