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This is an alternate solution to the replacement front for the 75mm filter holder, it is simply a compact step down ring that matches the internal thread on the existing polariser the particular British maker of the 75mm filter holder system makes.
What you do is you unscrew the retaining ring, carefully remove the polariser element, and then screw this adapter ring in it's place. From there you can then use any old 77mm filter.
This is here mostly for documentary purposes, if it were to be available it would be machined in aluminium as plastic isn't really strong enough for constant use.