Butter kneaded in browned flour is a brown beurre mani , an influence of French cuisine. Browned flour and beurre mani are explained on the other side of this sheet. Catsup was originally an eastern sauce based on fish brine, that had become commercially available in Europe and North America. Receipts also appeared in early cookery books. Very similar in intense flavour, colour and texture is worcesteshire sauce.
Muscles are mussels.
Mrs. Johnstone was a fine Scots cook, but her affinity for fine French cooking is found throughout her receipts. It's an uncooked paste of equal amount so butter and flour, in this case browned flour for darkening the sauce. Turn it occasionally, that it may colour equally, and keep it for use. No salt is needed because the sauce is already strongly flavoured. Short of making your own mushroom or walnut catsup, worcestershire is an excellent readily-available substitute.