These displays are now readily available.  The Vishay Dale number for this display is PD-20D025-5 and a data sheet updated March 12, 2012 is available at:

www.vishay.com/docs/37093/pd20d0255.pdf

This part number is the exact one that I found on my MK-12D display and the data sheet states these are Replacement for former Narco Avionics 50401-1.

I found these parts in stock at Digi-Key at:

http://www.digikey.com/product-detail/en/PD020D02505S51

On June 23, 2012 they had 35 in stock for $135 each.  At this price I might buy a spare.