If it is a hole and you ever get down to the Twin Cities area, look me up and we can go to a good pick-n-pull we have here (corbomight@excite.com). The yard doesn't always have a great Volvo selection, but they always have at least a couple and the failure rate on those tanks must be low because they always seem to still be there when most everything else is gone. You will probably find some other handy parts too.

I always assumed those depressions in plastic items were for machinery to use to hold and orient them in the manufacturing process.

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Andy in St. Paul, '91 745 Regina 190K mi, '89 244 137K, '87 245 RIP

The "hole" you're talking about isn't a hole at all, just a deep round "depression"... There's a corresponding one on the bottom as well, actually. What the reason for this design is, I couldn't guess, but rest assured they're all like that.

Correct on all accounts. System should be pressurized to raise the boiling point of the coolant. A new one's not free, but... not really expensive, either. Try holding 220F water in a milk jug at 5-7 psi day in and day out, with 150 degree ambient air and a lot of vibration.

If it's a small hole, then I would try an epoxy repair, and pad the offending strap with some bicycle inner tube or similar so it won't happen again.

Certainly it should pressurise, and a failure to would cause overheating. It must be a very rare fault - I've never had one with a hole. I used to have a shelf full of spares I was keeping for no obvious reason, but I've thrown all but one away now. Wrong side of the Atlantic for you though anyway.
I'd try a scrap yard. Check the pressure cap too - I have had several fail.