The meat paste sandwich is built around a jar whose contents are deliberately unspecific, and that ambiguity is the point of the form. The paste is a cooked, cured, finely pounded meat mixture, smooth and salty and spreadable, sold shelf-stable in a small glass jar under names like Shippam's or Princes, and what kind of meat it is matters far less than what it does. It keeps without refrigeration until opened, it spreads thin, and a very small amount carries a whole sandwich. This is thrift logic in its purest British form: not a generous filling but a concentrated, ambiguous, long-life savour that turns plain bread and butter into lunch with the barest possible scrape.
The craft is entirely in the ratio. Meat paste is strong and salty and slightly tacky, so it is spread thin, a scrape rather than a layer, and bridged to the bread by butter that carries the salt across and stops the paste reading as a smear rather than a filling. The bread is soft and plain because the paste brings all the flavour and has no texture of its own, so any crust with real chew would be doing the only work in the sandwich and doing it against the grain. There is no acid, no crunch, no second component by design; the entire effect rests on salt and savour against soft, buttered bread. Cut thin and pressed, it is a sandwich engineered for a lunch tin and a small budget, not a plate.
The variations run along the same shelf and the same logic. Beef, chicken, ham, and the fish pastes each carry a different cure under the same potted ambiguity; the bloater paste pushes a smoked-herring intensity; sandwich spread is the tangier vegetable-and-mayonnaise cousin that sits beside it in the cupboard. Each is the same scrape-thin shelf-stable idea met in a different jar, and each deserves its own article rather than being crowded in here.