Someone on eBay has a very similar one for sale:

http://cgi.ebay.com/Mitutoyo-Digimat...d=p3286.c0.m14

Maybe he can tell you the voltage and polarity of the "wall wart".

I think that Richard is probably right, but you could also pull the
back off the indicator and see if it has an old NiCd battery pack
inside. If so, you could trace it to the charger plug and guess
voltage/polarity.

I've used something similar in the past and it had an LED display.