It appears that for the Ricoh GR IV Monochrome they're extending the HDF Maker Note field to indicate if the Red Filter is in use.
R0000073.DNG: 0x009e = 2, Red Filter ON
R0000074.DNG: 0x009e = 0, Red Filter OFF
Checked on ExifTool version 13.59
DNGs are available if needed.
It appears that for the Ricoh GR IV Monochrome they're extending the HDF Maker Note field to indicate if the Red Filter is in use.
R0000073.DNG: 0x009e = 2, Red Filter ON
R0000074.DNG: 0x009e = 0, Red Filter OFF
Checked on ExifTool version 13.59
DNGs are available if needed.