The only Islay Single Malt from Signatory distilled in 1976 that I know is a Port Ellen. The dumpy bottle is in a blue velvet box, the whisky is 22 years old and at 55.9% alcohol by volume. I bought this bottle nearly two years ago for (now) 95 Euro and have seen it again a few weeks ago for now 165 Euro.
Either way its worth collecting and drinking.
Look at www.whisky-files.com
for more bottlings from Signatory (only Ardbeg and Port Ellen) in the case it´s not that bottling.