FIX Stream specifier ':a:0' in filtergraph description matches no streams with FFMPEG
ffmpeg -i "L.mp4" -i "reversed.mp4" -filter_complex "[0:v:0][1:v:0]concat=n=2:v=1:a=0[v]" -map "[v]" -c:v libx264 -c:a copy REVERSED.mp4
ffmpeg -i "L.mp4" -i "reversed.mp4" -filter_complex "[0:v:0][1:v:0]concat=n=2:v=1:a=0[v]" -map "[v]" -c:v libx264 -c:a copy REVERSED.mp4