In the early years of photography, a lens could be fitted with one of a set of interchangeable diaphragms [1], often as brass strips known as Waterhouse stops or Waterhouse diaphragms.
The problem is even worse when your barrel has no stops as is often the case with a lens which once had Waterhouse stops, now long since lost.
A lens cap was removed and then replaced to begin and end the exposure—a primitive version of a shutter. This vintage camera is surrounded by waterhouse stops (apertures) and a lens cap (the shutter) leans against it.
 See also: Aperture, Image, Stops, Lens, Photography
  
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