I have a tap for that plug ,will have a look at what it is.Time for a thread gauge.
Metric & Imperial & US Thread Screw Pitch Blades Gauge Gage Measure Range | eBay
Knowing pitch and at least rough OD should find it for you.
My spare HIF6 manifold doesnt have that plug, so I cant help you.
I will have a look later.I was going to use the inlet to hook up a vacuum gauge. If I know the thread I could see what might already be available commercially. Its laziness really, I could take the blanking plug out and take it to my local engineering place to check next time I'm going that way. But if someone already knows what the thread is.....