There were no commercial LED displays, of course, so the
results were displayed using incandescent lamps to side-
illuminate numbers cut into polycarbonate blocks! (I don't
know why NLS didn't use Nixie tubes.)

HP made a voltmeter plugin for their 5245L frequency counter.
That was mid '60s vintage.

Interesting. I had an SWTP modular unit, though that was a lot later.

The NLS appeared earlier, I believe. George Kay (as in Kaypro) is given
credit for the first digital voltmeter.