Carl,
Once the pusher plate has been seated, the short connecting tube is inserted into the plate hole; the tube would then be sealed with a rod insrted into the hole to keep out the hot matrix.
With this done the matrix and balls could be filled; after the matrrix has solidified , the fuze body hole enlarged and the fuze screwed into place with a gasket to seal around the fuze head.
Please remember this is speculation on my part. Perhaps another member has a reference to documen the procedure.
I have noticed that in most case shot, smaller balls are used near the top of the matrix and noticed that the matrix may not be completely full to the top.
Regards,
John
P.S. Notice the cut nail is missing on this specimen as the shell was cut at a slight angle cutting off the nail in a vertical position.