This subject comes up regularly.http://goscf.com/t/55976
Note the post by Angore:
For Machins, It is not an easy task to correlate Scott to SG as SG lists so many more. One Scott number often is multiple SG numbers because Scott ignores tagging. Now Scott does list thick/thin values and not all these map to an SG Concise number.
And this is just if with SG number you mean SG Concise numbers. SG also has a specialised Machins catalogue that has even more numbers and a numbering system that is different from their ordinary "SG" numbers.