A well looked after specimen of a Stuart 504 water tube boiler, largest of the 500 range. Copper silver-soldered construction, 60psi working pressure. The boiler was originally supplied with a primus type burner, pressurised from a bicycle pump and running on paraffin. I've seen these paired with other Stuart boilers, although never a 500 type before. Best bet by far is to replace with a modern ceramic gas burner, which brings a welcome degree of control to steam production, whilst also burning very quietly.
Further details about all the 500 series boilers are filed away on my Stuart page.