Hi again,

I looked around EBay because they often have old stock at good prices. I noticed an old style Kats that looks to have a brass wingnut and screw.
There's a Kats 11604 for $18.00 that fits your 44.5 mm bore, but it has an outer flange that may not allow it to go deep enough. Their application guide is no longer listing Volvos before 1985.

If you find one,

Apply minimal torque to the screw. The O ring on straight bores seals on the sides of the hole and the wingnut only retains it from blowing out. It doesn't increase pressure on the O ring, as it does on the stepped bores used on the B230 85- mid 91. Maybe that's why they changed, but it doesn't explain why they changed back.

Peter