I just saw this great research on The Ring look
Based on this video i seached for the filter described there.
Then i found paper about the transmission values of all CC Filters for each wavelengths
In the video was meantioned that the Filmmaker used a 81 EF Filter with a green Filter
I have tested both Filter and come to the conclusion that the Filmmaker was using a cyan Filter instead of green.
Here a screenshot from my testing.Have not played with contrast or something just the two Filter.81 EF and cyan CC 50
For comparison the Filter from the video researcher used (CC30 green)without 81EF
And (CC30 green with 81EF)
To me cyan cc50 with 81EF looks the overall closest.