I thought this was interesting.

I know exactly what you are describing, and that system is indeed called an overflow bottle rather than an expansion tank. The two ideas are confused because they both add a plastic bottle to a cooling system.

In the overflow system, the pressure is maintained by the special radiator cap designed to permit much higher system pressures and take the overflow back in on cool down. Just as you stated, the bottle is not designed to handle that pressure.

With the expansion tank, the system pressure is maintained by the cap on the bottle(tank) -- the only type I have ever seen on any Volvo. That tank is designed for the pressure. To me, it seems the more highly evolved design, but I can envision the argument against my Volvo bias.
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Art Benstein near Baltimore

"Carefully pry (If this were a Jaguar you would prise)..." -BrickDad