The aforementioned recipe from https://archive.org/details/saladsandsauces00murrgoog/page/n... says:

> Caviare is the roe of the sturgeon prepared under many formulas, the Russian being the best. Take a teaspoonful of caviare, put it in a soup-plate ; add to it a saltspoonful of chopped onion, a walnut of butter, and the juice of half a lemon ; work well together, spread on thin slices of rye-bread, press them together, cut the sandwich in two, and serve. Americans as a class do not like caviare.

According to Wikipedia, 1 salt spoon (ssp) = 0.25 teaspoon.

The other recipe book has caviare-lobster sandwich (species unspecified; I assume sturgeon), roe sandwich (salt herring), and two types of shad-roe sandwiches (shad). See https://archive.org/details/uptodatesandwich00full/page/12/m... .

https://en.wikipedia.org/wiki/Caviar mentions:

> In Scandinavia, a type of sandwich spread is available, made from smoked cod roe and other ingredients, which is referred to as smörgåskaviar (meaning "sandwich caviar"). Outside Scandinavia, the product is referred to as creamed smoked roe or in French as Caviar de Lysekil.

Perhaps the Swedish term "kaviar" doesn't overlap fully with the English term "caviar"?

https://en.wiktionary.org/wiki/kaviar#Swedish says:

  1. caviar (roe of the sturgeon)
  2. creamed smoked roe, Scandinavian sandwich caviar

  1. Roe of the sturgeon or of certain other large fish, considered a delicacy.
  2. (figuratively) Something whose flavour is too fine for the vulgar taste.