Good for you - on your plans on collecting the half sovereign.
NGC has the slots correct. The IOM came out with the 1965, they then started the series in earnest in 1973. There are only proofs for 1983. Pobjoy confirmed That they did not strike any BU 1983 1/2 sovereigns. I've started to collect the die varieties as well.
One interesting occurence. I saw a 1993 IOM 1/2 sovereign with the Norse warrior reverse for sale. It was a bit pitted, but overall in good shape. I was after a couple other coins at the time and did not buy it (I should have). I talked to Pobjoy and they could not determined if they issued it or not (again very poor records). They said it is possibly theirs that was made special for a dealer, but weren't sure. Who knows if another one will pop up and then the question will be to add it into the registry or not. Without proof the Pobjoy issued it, there will be a good debate.
I've talked to Southhall a few times and folks at the Pobjoy mint many times. Pobjoy drives me nuts with their inadequate records. Southhall did a good job with the information out there and at the Pobjoy mint. He does have some errors in his book, but overall - it is good and the best reference in existence. I continually look for these issues and will probably upgrade a couple in the future. Let me know when you start - it is a fun series.