No, i would have thought that it's an 1 1/4", shimmed to 1 1/8" when 1 1/4" became hard to find.
Do you have verniers? Can you measure the outside diameter of the head tube on the frame?
Wall thickness is going to be around 2mm, so subtract 4mm from this measurement.
If you have a spare 1 1/8" head set see how far off the cup size you are.