I play HiLo. For 6D,S17 bj and for 6D,H17,DDD Span21, I find the following plays have the same index number.
1010-6,1010-5,16-10R,15-10R,15-9R,A8-5,1010-4,A8-4,15-4,13-6,14-6,11-10,11-8,10-8,A7-3,A7-4,A9-6,A9-4,14-AR,16-8R. Now isn't that interesting for such different games to have 20 indexes in common? I created this list because I would stop when playing Span21 when I hit the same index, thinking, no no I'm making use of regular bj indexes. My brain now has learned to accept this.