I've had a few flaky ones do exactly what you're suggesting - plug them in, and another display goes awry.
When I bought my Sys3-6 test fixture last year, the seller had 75 NOS 6 digit display glasses, and sold them all to one buyer at VERY reasonable prices. I'd have liked to have had enough to replace all the glass in the fixture, but the guy didn't want to sell any of the ones he bought.
New glass is out there, but you have to be patient.
BTW, I love your avatar! Would you believe I'm a HUGE Get Smart fan?
Thanks, Get smart nearly 50 years on & still cracks you up, going through the remasterd DVDs, I think over 130 episodes,
