We have the confirmation from Eikon Content team that "CTBTR_1 or FID 831 will be populated with the contributor's name if the RIC is a contributed RIC. <BBBBNK20YCD=R> is a composite RIC, hence REUTERS EOD is displayed". However when we tried to return the data for CTBTR_1 field of RIC BBBBNK20YCD=R, nothing returns.
not sure what is product service package you are using to access this FID. If you are on TRKD packages via Elektron Connect then this FID is not supported. I strongly believe your entitlements are associated with TRKD hence you do not see this field. Alternatively you can try using this field CF_SOURCE or 5369.