Wide wheel rims

Does anyone have the SIP wide rim conversion?
I’m a engineer by trade and I keep looking at the conversion kits SIP sell and they look like they’d be pretty easy to make.

The only questions i have are:-

I take it you put the spacer between the rims and then mount the rim the opposite way than normal? Is this correct?

where does your inner tube valve exit? does it just come out between the rims?

And in the photos of the SIP kit it shows a counterbored boss, whats this used for? I can’t see its purpose but i’m probably overlooking something simple!!

Many thanks