***
Hi Nick.
There is not much SG information in the public domain. I put a chart with information on a range of Old English makers into an article I wrote in The 2007 PCA Bulletin. George Kulles published a table for a range of antique paperweights in his 1987 book on Identifying Antique paperweights - Lampwork, but I think there are doubts over the attribution of a number of these pieces (not least by George himself!).
In general terms, lead glass that was 'full lead crystal' had at least 24% lead oxide in the batch, which gives an SG of around 2.85 (it is not possible to be precise, because it depends what else goes into the batch besides sand), and most antique lead glass paperweights I have measured have been 2.90 or higher. I believe that some antique makers used 33% or more lead oxide for general crystal glass, giving an SG as high as 3.45. Most non-lead based glasses (Murano, China, Caithness, Ysart, Perthshire, Deacons etc etc) are in the 2.45 to 2.60 range.
I'll see if I can put my current Old English results chart on line somehow. It is not secret - just a rather large unweildy thing in its present form.
Alan