Hi Eddy. Interesting weight and results. I understand how you are estimating the specific gravity, but I think there is a risk of significant experimental error - however, maybe you have tested the same weight several times using your method, and knows how large or small that is.
I carry out specific gravity measurements using the weight in air / weight in water method, using a pivot / beam / knife edge / and electronic kitchen scales (similar to the setup Derek uses). I measure the paperweight in air, then without changing any aspect of the measuring system, I bring up a container holding water from below, to immerse the paperweight. Those are the only two measurements you need. I have discussed in detail the various experimental measurement sensitivities in an article in the recently published PCA Bulletin 2007. Apart from accuracy and repeatability of the equipment, the next most important thing is to work at constant temperature - the water and weight need to be at the same temperature (for example, 20 deg C). Water purity is of very little importance.
I have measured over 150 Old English weights, but have not measured any VSL or Clichy yet - I will do so in due course.