I also found this:
http://www.pdp8.net/tu56/tu56.shtml
in which Dave G. describes a successful use of the CM2182 (14V,
80ma) bulb in this application.
I just replaced one of them again last week so I had taken a picture of
the indicator disassembled.
It is now up at
http://www.pdp8online.com/tu56/pics/p1000839.shtml?small
If your TU56 -15V input is high you may run this bulb above its rated
voltage shortening its life.