not for certain, unfortunately.
However, quoting from Susan Tobin's book..................
""In the mid-1970s though the existing number for current items remained the same, the alphabetical code was changed from RSW to SG. Additional codes - CG, CJG, SJ (Silver Jubilee of Queen Elizabeth's accession - 1977), SJG, and ST - were introduced from that time also.""
The longhand explanation for some of these codes would appear to be fairly obvious........... i.e. Commemorative Glass, Commemorative Jubilee Glass etc., but Susan Tobin doesn't explain what SG stands for.
In the various Appendices in the author's book, other codes also look to be obvious ............ B for bowls, D for Decanters and related objects, J for jewellery, L for lamps etc.
Might SG have been for Stennett Glass, Sandringham Glass, Small Gifts, Small Glass etc.
Perhaps someone should send the lady an email and ask.