The RS274D are standard Gerber files in plain text that contains X and Y coordinates. You can practically open them with any text editor and supported RS274D application. I've found an application that can successfully read RS274D files. It's called ViewPlot and available for download on the application's page at Software Informer. Press the big green download button to initiate the transfer of the file. Of course, GerberLogix can be used as well.