The method I use to remove this stuff (documented many years ago
in an AMICA Bulletin Technical Article, and probably long-forgotten)
is as follows:

I modified an electrically-powered, hand-engraving tool, the type that
has a vibrating, usually carbide or at least hardened conical shaped
point.  On my tool, this point is replaceable and held in place by a set
screw.  I removed it and VERY carefully ground the opposite end into a
flat chisel shape.  This is best done with a fine "green" grinding
wheel, but can be done with a regular carborundum wheel as long as it
is fine and you go slowly and avoid building up heat in the tip.

Placed back in the tool with this end outward, you now have a miniature
"jack hammer" that fractures this poured material quite nicely.  Avoid
having the tool tip come in contact with the tracker bar nipples as it
can puncture them or push them out of alignment if you are not careful.

Once the cement is removed, and remaining lead segments can be scored
with a sharp "box cutter" (trapezoid shaped blade) and then twisted off
with needle-nose pliers.

My tool has a 5-position switch whose original purpose was to vary the
power delivered to the engraving point.  I typically have mine set on
"3" for this task.

John Grant