These Amerex commercial Direct Pressure extinguishers utilize a 23 ft3 nitrogen cylinder with a “quick opening brass valve” to pressurize and completely discharge the agent cylinder. The compact size and 16" semi-pneumatic tires allow easy maneuvering and operation by one person, through narrow doorways and aisles. A 4" stainless steel fill opening with rugged chrome plated brass cap makes servicing easier. For large hazards choose the 300 lb. ABC or Purple K or 350 lb. Regular Direct Pressure extinguishers with a “quick opening” 55 ft3 nitrogen cylinder. They provide quality and economical fire protection, large volume capacity for extra high hazard areas with single person transport and operation. The 36" X 6" wide steel wheels make movement possible in sandy or soft soil.

http://www.safetyemporium.com/09564

125 lb. ABC & Purple K and 150 lb. Regular Direct Pressure extinguishers utilize a 23 ft3 nitrogen cylinder with a “quick opening valve” to pressurize and completely discharge the agent cylinder. The compact size and 16" semi-pneumatic tires allow easy maneuvering and operation by one person, through narrow doorways and aisles.

https://www.amerexfire.eu/media/14256/Amerex-Wheeled-Extingu...

In terms of design, a distinction can be made between fire extinguishers under stored pressure (where gas is placed directly in the containers) and extinguishers with a cylinder (where gas is placed in a cylinder within the container).

https://pastor.hr/en/production/fire-extinguishers/powder

Slide 13
Tipos de Extintores:Tipos de Extintores: SEGUN LA FORMA DE IMPULSION :SEGUN LA FORMA DE IMPULSION:
Presión Directa o Interna.
Presión Indirecta o Exterior.
Por Reacción Química

https://www.slideshare.net/carlosparra201/extintores-3255259...

Dry powder extinguishing agent which is forced out of the extinguisher under pressure by a non flammable gas.
Contains 9.0Kg of High Performance Dry Chemical Powder

https://fireproductsdirect.com.au/9kg-dry-powder-fire-exting...

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Note added at 2 hrs (2019-07-22 15:44:43 GMT)
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DRY CHEMICAL - extinguishers are either stored-pressure models or cartridge-operated models. The stored-pressure models have a lever above the handle for operation. The cartridge-operated models require two steps: Depress the cartridge lever, and then squeeze the nozzle at the end of the hose. The dry chemicals leave a residue that must be cleaned up after use.

http://www.americanfireandlifesafety.com/different-types-of-...

As far as I can see, 'direct pressure' is also known as 'stored pressure'. I'm not an expert in fire extinguishers, though I had a couple for over 20 years that used to be serviced every year and emptied and refilled ('retimbrado') evey five years.

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Note added at 2 hrs (2019-07-22 15:53:33 GMT)
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Stored Pressure Fire Extinguishers
Stored pressure fire extinguishers contain the extinguisher agent (water, powder, foam, CO2, etc.) and the pressurising gas (dry air or oxygen-free nitrogen) in the same cylinder.

Cartridge operated Extinguishers
Cartridge operated extinguishers contain the extinguisher agent (water, foam, powder, CO2 etc.) in the main cylinder (this is not pressurised) and a seperate gas cartridge containing highly pressurised CO2. Once the handle is squeezed the CO2 cartridge is pierced releasing the pressure into the main cylinder.

https://www.safelincs.co.uk/guide-for-servicing-and-maintena...

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Note added at 2 hrs (2019-07-22 16:16:05 GMT)
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I've been reading about fire extinguisher mechanisms and I think 'stored pressure' is the right term. I'm sure it means that they're immediately ready for use by pressing a handle and lever. 'Cartridge operated extinguishers contain the expellant gas in a separate cartridge that is punctured prior to discharge, exposing the propellant to the extinguishing agent'.

https://en.wikipedia.org/wiki/Fire_extinguisher

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Note added at 1 day 19 hrs (2019-07-24 08:41:26 GMT) Post-grading
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Jane, thank you for the points. I have to go into town later and out of interest I'll ask the person who used to 'look after' my fire extinguishers what it could mean.