I used another NVenc tool that allowed me to dump capabilities of my card:
#0: GeForce GTX 950 (768 cores, 1329 MHz)[PCIe3x16][442.74]
List of available features.
Codec: H.264/AVC
Max Bframes 4
B Ref Mode yes
RC Modes 63
Field Encoding yes
MonoChrome no
FMO no
Quater-Pel MV yes
B Direct Mode yes
CABAC yes
Adaptive Transform yes
Max Temporal Layers 0
Hierarchial P Frames no
Hierarchial B Frames no
Max Level 51
Min Level 1
4:4:4 yes
Min Width 33
Max Width 4096
Min Height 17
Max Height 4096
Multiple Refs no
So, it looks like my 950 has Ref Mode but NOT Multiple Refs. And this works in only one way:
* leave Number of Reference frames undefined (should be able set it to 1 but even ffmpeg errors on that).
* Set Use B frames as references = Middle
Output now shows:
FFmpeg: : Video: h264 (h264_nvenc) (High), 1 reference frame,