The older model is behind the later version. You can see the slight difference in the shaft thickness and the casting of the name. An early model “notch-less gate” Pierre Allain carabiner provided for scale.
Left: The newer model hook
Right: The older model hook
The stock where the shaft transitions into the forked end is thicker on the older version. The shaft itself is slightly thicker.
Top: Older model
Bottom: Newer model
Left: Newer model
Right: Older model
The older version is slightly thicker and less refined.
The eye of the older version Pierre Allain Descender. Notice the P. Allain name.
The eye of the newer version. Pierre Allain is spelled out completely. The outer diameter of the eye is the same as the earlier version but the inner diameter is a bit larger.
Detail shot of the forks. The later version (bottom right) is “sharper”
Top: 1968 model
Bottom: Pre 1961 model
The reverse side of the Pre 1961 model Pierre Allain Descender.
Detail showing the small difference in the clip in eyes of the two versions.
Not much difference in the forks of the descenders. The later version had slightly sharper tines.
Detail of the eye of the pre 1961 model. You can see some manufacturing defects inside the eye.
Top: Pre 1961 model
Bottom: 1968 model
Top: 1968 model Pierre Allain Descender
Bottom: Pre 1961 model Pierre Allain Descender
An early style Pierre Allain carabiner clipped to the Pre 1961 model Descender.