Pictures and info submitted by Renosteinke:
When the PoCo went to disconnect service for non-payment, the meter ring came apart, leading to a service fault to ground.
The meter base was probably made just before WWII. Manufactured by Square D, the box cover had little slots, into which tabs on the ring were press-fit. Unfortunately, the meter socket was also attached to the ring- so when the ring came forward with the meter, the socket also cam forward, contacting the box.
At about 2 O'clock on the cover, you can see a round notch in the cover, caused by this arc.
Farther down the line, you can see the scorch mark on the EMT, caused by this fault arcing between the two pieces of EMT.
Best I can tell the meter-puller had to stand there, in the winter cold, confronted by hostile non-English speaking tenants, holding the meter in place- while waiting for assistance to come, and disconnect power at the pole!
Then I got to do a service change!