The cap is probably the filter for the 200 volt supply. They went
left & right on ALL brands. change to a 450 V & higher temp.
they cause all sorts of poor pix problems. Never seen anything blow
one out but it is possable....
Doug covered the oil cap well. You can also get one from almost any
Zenith solid state up to apx 1979. Other brands used it too on
top of line chassii.
If cap shorts set will hum loud then pop breaker. If it opens the
pix will "breath" or pulsate. Rarely went in old days but common
now.
BTW your set uses an EFL CRT. Not many seen here until later years.
In J line it came in 17 & 19" only on high end sets.
73 Zeno
LFOD !