I'm certainly no expert and have no experience with this particular carb, however, after looking at the diagram in the link posted by bruce4310tx I suspect that may be a constant velocity carburetor. Further increasing my suspicion that this is a CV carb it appears that an enrichener is used to enrich fuel mixture for cold starts instead of a choke plate; enrichener systems are commonly used on CV carbs because they function better in providing a richer mixture for cold starts in that design and remove the restriction of a choke plate allowing increased air flow and better performance over choke type carbs. The plugged tube may be a vent tube designed to allow air movement above the diaphragm under that metal cover to move in and out as need be. It may have originally been connected to a filter of some sort that prevented water and dirt from entering while allowing free air flow, or to a manifold vacuum source.
If it is indeed a CV carburetor there should be a slide (could be flat or round) valve, with needle attached, beneath the diaphragm that controls air flow dependent upon vacuum generated when the throttle valve is opened. The use of a CV type carb is a possibility because CV carbs have the ability to compensate for altitude changes without rejetting, a desirable feature for a vehicle which may be operated in a large variety of altitude conditions.
Unfortunately the diagram doesn't really break the carb down far enough to see the internals so it's type, function and operation remains open for consideration. Hopefully others more qualified than myself will be able to be more precise.