I came across an information that the rear element (sub-cell) of the front Heliar cell can be unscrewed till 1.5mm and this can be used for adjustment of softness effect...
I coud not confirm this "revelation", and never heard about it. Did you, or someone tried it? I checked all my Heliars 105/120/135/150/165mm and last three have it.
Here's a picture made by Uusilehto which shows it:
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