What engine is this valve from?

Someone has asked me if I can identify the larger of the valves in these two photos…and I can`t, so over to anyone else who can?
The smaller one is Merlin. The head diameter of the unknown one is 2.5″, and the stem 0.465″. It has a rubber o ring seal just under the spring and the guide appears to have been retained by a nut.

Cheers

Pete

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