[HECnet] DECmate II and OS/278...

Ah! Teledisk images. Basically, that's a proprietary format which is intended to be able to restore the disk including the low level format. It's a bit difficult to deal with if you are not using Teledisk. I know I have a tool somewhere that can extract the actual data from such an image, which is what would be more useful when you are writing it on an actual RX50 drive. I believe I found it somewhere on the internet many moons ago, but hopefully others have it too.

In addition, RX50 drives in general only accept preformatted media, and RX50 low level format is unlike any other disk format, so you either need real RX50 floppies, or else some machine that is able to format RX50 floppies. Of DEC equipment, only the Rainbow were actually able to format RX50. Generic PCs can sometimes do it, if you have the right type of controller and drive, and appropriate floppies. But success with that have been a bit random when I've read about it. But I suspect it's your best chance, since getting genuine RX50 floppies nowadays is even harder.