I don't think so, I was suggesting that duranash attempt to import the original scanner RGB data so that, in the absence of a profile, a working space can be assigned. In this case it sounds like the software is clipping the data to the gamut of the display and sending the data as untagged, adding to the confusion.
Jacopo is correct, the data isn't in ProPhoto RGB and assiging it isn't going to make it so and in fact will likely kill the color appearance. You'll have to convert from Scanner RGB into ProPhoto RGB or just leave the scan in Scanner RGB as its likely much smaller in terms of gamut than ProPhoto RGB. Plus if R=G=B is indeed neutral, might as well leave it in the original color space. In Raw workflows, especially in the case of Adobe, the processing color space is ProPhoto chromaticity so it makes perfect sense to just encode the final data in ProPhoto RGB.