I noticed that GX21's pipe look like my LST's pipe, dimension wise. No evident differences. On the other hand, they both looks completely different than my Tamiya's .15 pipe which is visible smaller. You are probably right when you say they are grouped by block size. Well, except the ones that you pay $100+ for I suppose I'll buy that $16 set pipe and manifold (since I'll need one anyway) when the time will come