If it were really meant for 240V I don't think it would work on 120V. Also you have a 120 type plug correct?
It's possible they put a 120V sticker over where you now see 240V and it fell off over the years.
Is there a mains fuse? What is it labelled as and what is it actually? A 120V amp would use a fuse twice the size (rated in amps) as a 240V model.