At the time of that design, it was common to use paper or oil-filled paper capacitors in the HV filter after the rectifier.
The good ones were hermetically sealed (sometimes called "bathtub"), since paper is hygroscopic.
WWII military equipment often required this construction (paper sealed inside metal), and Heath used a lot of military surplus components in their early equipment.
I would replace that sealed multiple capacitor unit with three modern polypropylene film units, that should fit in roughly the same space, with appropriate terminals.