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700/900 red blocks don't have that half moon disc as they have rear cam distributors. If your flame trap and all attached PCV hoses are clear then suspect a plugged oil evaporator box (that big black thing that the main PCV hose attached to down at the base of the block). You could try attaching a hose and blowing into it with the oil filler cap removed to see how badly it's blocked, but if it hasn't been cleaned/replaced before then just assume it's getting plugged after all these years.
Do not try cleaning it in place. Removal and cleaning (rodding and flushing) often suffices, but many will simply replace it, also it does get brittle with age. Rather awkward and messy to get to, especially if an EGR valve is in the way. Be careful not to poke the plastic drain tube in the block below it down into the oil pan. If an EGR valve is preventing access (such as attached to the front bolt), you will need penetrating oil, larger wrenches using counter hold methods and patience to get it out of the way, maybe a couple of band-aids for your knuckles. It's often simplest to disconnect the outlet tube at the intake manifold.
If your PCV system is clean you shouldn't need a metal retaining strap (otherwise Volvo would have put one there), but it's always safer to have one. You can easily make your own strap, you just need to get appropriate metric bolts for the unused distributor holes at the back of the block, likely 10mm if I had to guess. You should be able to find what uou need in decent hardware stores, often in the bulk small fastener bins, and as well in auto supply stores. An aluminum strap works well as it's easy to drill and won't rust, thickness doesn't really matter, 5 layers of a foil pie plate would probably work in a pinch, no one will see your cheap handiwork.
I never had one blow (as I always maintain my PCV systems), but I've witnessed a couple of people who have, one where the plug was found and one where it was lost on the highway.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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