What was the purpose of those sealed condensers? Was placement in a particular circuit that critical for such a sealed capacitor?

Brian: I believe the idea was to deter degradation of paper caps by sealing them in tar. It didn't work, though.

Maybe a marketing ploy, too? Might have deterred repairmen from using generic replacement caps?
Doug

:What was the purpose of those sealed condensers? Was placement in a particular circuit that critical for such a sealed capacitor?