Here is a link to the website of Old Custom Cruisers, a car club in Bristol, Tennessee which owns two Ahrens-Fox pumpers; 1 from Chicago and 1 from Maywood, Illinois. 

1925 Ahrens Fox Model JS4 750-GPM pumper

Hours on Pumper: 750 gal @ 120 lbs for 6 hrs, 375 gal  @ 200 lbs for 4 hrs, 250 gal @ 250 lbs for 2 hrs

Engine details: 6 cylinder, 2 plugs per cylinder, duel ignition system, magneto & distributor, mechanical brakes 3-speed manual spring suspension.

The “Ball”, or the pulsation chamber, reduces  the throbbing of the stream as it ejects from the nozzle. The tall chamber on the front retained  the vacuum on the intake side of the pump.  With minor variation, the style of this pump did not change from 1915 to 1952.

Served in the Maywood, Illinois fire department.  Manufactured by Ahrens Fox Cincinnati, Ohio

former Maywood FD Ahrens Fox pumper

Old Custom Cruisers photo

former Maywood FD Ahrens Fox pumper

Old Custom Cruisers photo

thanks Dennis