Argus 2 - that is the front element of the lens, it is not a plain window.
On Argus2 we had a durable outer coating on the front element, which had to be fixed, and focus adjustment only used the rear element
Argus3 has a window but it is a larger diameter.
It is the same lens design as Argus2, but with 'internal' high efficiency coatings on all faces, only the window has the durable coating - and then on one side only.
Mainly down to mechanics and build, as Argus2 had to accept a lot of the decisons from Argus1 - where the tube moved to focus it.
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