This appears to be correct after testing. As EarthQuake mentioned, it's a red/green only compression format "BC5_SNORM". Making the blue channel white makes all of the grey/red/green images look like standard normal maps. Thanks all
In this case I don't think even that's needed, it's just assumed the blue channel is white (set the blue channel to white in the top image and you get much the same output as the second image) with tangent space normal maps there's never really a lot of detail in the blue channel. All the normal perturbation is in the red…