Hello Stephen,
You are quite right, a large number of QSA's are missing clasps and as you say, they do tend to be date bars.
In the case of the SA01 and 02 date bars, these tend to be issued of the supplementary roll, so a fellow has recieved his QSA and then, in, say 1907 is issued with an additional date bar.
In the British army, that last bar was often worn just slipped over the ribbon, but just as often, it was left to a skilled artisan within the regt/corps to rivet the bar onto the preceding bar, like this
or the soldier concerned might take his medal to a jeweller and have it fixed, but, I have a number in my own collection that have been sewn on or attached with wire etc.
The Gordon Highlander's had a bar on the India Medal 1895 fixed with a very distinct wire loop which is unique to them, and very nice to see today!
My own thoughts are, in the case of just a date bar, slip it over the ribbon or just leave it alone!
With your GG's medal, I would say you are very lucky and you should leave it as is!
You can easily obtain original date bars and I would just slip the bar over the ribbon, in a case where you believe that the bar was actually issued.
If you know that a man was entitled to an additional bar but there is some doubt that he actually recieved it or ever wore it, then I really would not worry about it!
Frank