The problem is the softening to an unacceptable degree of the herring flesh after six weeks refrigerated, in-jar storage. Briefly, the current manufacturing method is cut herring into chunks, de-salt in gently running ice water, drain overnight, pickle in a spiced, sweetened vinegar solution for one week, drain herring, fill into jars, add a small quantity of dried spices and freshly sliced white onion, top up with fresh pickling solution and apply lids. Any suggestions as to texts or references I could consult or other comments or suggestions would be really welcome. It could be because you do not have sufficient salt in the product.
Acid softens the texture and salt hardens it.
You will find manufacturing procedures and recipes in some texts on fish processing and in articles, but you might have to go back some years. For example, 'Fish as Food', edited by Borgstrom. Still probably the best source of information on 'traditional' fish processing. These are the controlling factors for texture.