That is the zither attachment you are referring to.  A few Mira owners
that I know (including me) remove that attachment in order to hear the
full capacity of the box with no altering of the sound from the combs.
The "zither attachments" I have removed from my Mira boxes appear to
have originally had a soft gray material running the length of each
zither.

Regarding the disc being stuck, first check the disc projections to
confirm that there are none that are bent or otherwise not in alignment
as the other projections on the disc.  Mira discs have very sturdy
projections so if you try to re-align a bent one with needle nose
pliers it may snap off anyway.  Better to have no projection (and
therefore no note) where there was a badly bent one than having a bent
projection catch on a star wheel and potentially cause more serious
problems with the gantry.

The 18-1/2" Mira motor packs a lot of power and when a disc gets hung
during play it's good to remember that the disc still has a lot of
tension on it if you are considering removing the disc.  If you push
the disc as you indicate that you have to get it past the hung
condition you may be introducing more tension on the affected star
wheel(s) than they were designed to sustain thereby potentially causing
a serious injury to the star wheel(s) or the gantry itself.

If disc projection(s) are not the cause of your disc locking up then
check the disc drive holes at the edge.  If they're okay then make
certain each of your star wheels are turning freely.  If any are not
then worst case scenario you may need to remove the gantry shaft and
star wheels to clean and check and re-install each of them as they can
get stuck good and tight with ancient gunk sometimes.

Been there, done that and it is not a fun or simple job.  Hopefully you
will find that a couple of disc projections are the culprit.

Those are a few quick ideas that may help.  Good luck.

Doug Olds