Two types, three-tier and four-tier. The oldest model is four-tier having individual screw-on pneumatic-valve units. The older type has a rather unique type of air motor to distinguish it; it employs metal pistons and distributor valves instead of the conventional wooden construction with bellows cloth.
The later models: Mostly glued-together stacks, which are now tricky to rebuild, but certainly averted the problem of leaky gasket joints when new. One method of rebuilding them now is to saw the three tiers apart by ripping the upright board up to the pneumatic hinge, leaving the section of the chest attached, later bolting this back together with a gasket the width of the saw cut. On some of the later models, three rows of individual pneumatic-valve units were screwed on with gaskets, as in all of the early four-tier models. All three tier models a can be identified by the unusual pneumatic motor.
More information about rebuilding the Gulbransen is found at the following links:
Rows of screws on the front of the stack identify the Gulbransen with the 'Screwed' stack. If the screws aren't there, the stack is the 'Glued' type.
407 19th Ave, Brick, NJ, 08724
Phone Number 732-840-8787