Ben, et al: An LS-8 seat back will work. There's even a Tech note for
installing that part, TN6013. Buying a new part now requires that you
pay the ransom money to DG. Perhaps Tom Gilbert has something Ben can
use. But he has likely already spoken with Tom. The LS-4 seat back has
different lower mounting hardware to the LS-6.
The LS-6 has a fairly tight cockpit, so some people fly without the
seat back which is probably why this one is missing.
I did a modified "4U" (Harry Wuentstel) mod to #6046 seat back: Cut
the headrest off and bond it back on at a slightly more "tilted
forward" position. This allows the seat back to be reclined quite a
bit more, resulting in more headroom. If you have the flat LS-6A seat
back, you may want to do the mod. Of course, this is for EXP gliders!
Jim