The inner diameter of the collar is the same size as the original steering wheel hub for the horn button and the outer diameter of the raised part of the collar is what the steering wheel center fits into. I can't remember where I got this spacer, but it certainly wasn't meant for a 308. No matter, a little lathe time and it is a perfect fit.
The stepped collar is just press fitted into the spacer. The collar OD is very slightly bigger than the spacer ID and I don't think it's going anywhere.
I must say I'm pretty happy with the result. The spacer looks a lot better being the same diameter as the rest of the steering column and it feels great sitting behind the wheel. That extra 1.5" brings the wheel to a perfect position and my view of the instruments is better too. The next time I'm at a store I'll pick up some matt black crinkle paint and paint it to match.