this one came out of a Parasound A21 or A23 amplifier
they were used for bypassing the power supply near the output transistor.
They are white case 25mm long, 10mm thick for a 0.05uf/ 250volts
hard to see with the cellphone picture but it does say "full PSB" on it.
No more marking on any side.
Maybe John Curl could help here
As a signal coupling cap, they are terrific and I want to buy more.