RE: 5mm to 1/4"-28 prop adapter?
Yeah, you're probably right.
So, is this for a prop? I'm guessing not, because you need a specific size. If it were for a prop, just go with the flow and buy the nearest metric size (6mm), then ream the prop hub to fit.
Can you tap out the receiving end for the metric threads? Some careful reaming with a drill and a 7mm tap?