The black stuff is tar.
Today , most of this capacitors are leaking and suffer from great changes in capacity.
For restoration purposes, new capacitors are molded again in tar and they're looking the same as in those days.

This was the way Philips potted their impregnated paper capacitors from the 1930's on. They were made almost the same way throughout the 1950's and at least part of the 1960's. From the late 1950's on, longlife epoxy coated polyester capacitors took over partially. For high voltage, low loss, high dV/dt applications, Philips continued to use paper or mixed dielectric capacitors, though shifting to metal or plastic casings and epoxy potting over time. Starting in the 1970's, polypropylene capacitors took over for most of the remaining applications, though I think Vishay BC might still have a small series of MKT-P mixed dielectric capacitors for designers who insist on using them but got burned (sometimes literaly) by other brands.

While those tar encapsulated types weren't too bad as far as affordable paper capacitors go (when new they were better than even the few remaining current paper types - think RIFA), they should be considered 'replace on sight' if your goal is to restore an apparatus with the goal of using it daily. If they don't leak electrically at low voltage and room temperature, they will leak at higher voltage or after warming up a bit. A telltale sign is that even when cold, the measured capacity is often much too high.

The pitch coated capacitors aren't specifically meant for tropicalized radios - they were just 'the' standard Philips paper capacitor.

Out of all paper capacitors, it's one of the least bad types. But they're now well past their 'sell by' date. If you're only gonna turn on the radio for 15 minutes every month, you can leave most (but not all) of them in place. For a reliable set that you can turn on for hours at a time, all have to be replaced.

They're better than Wima Tropydur capacitors (the taffy or candy-shaped, often shiny capacitors).

There are vendors that sell unused pitch-coated capacitors - don't buy them. Even when you don't use them, the chemical process that degrades the paper continues.

There's one exception: using them in low voltage applications (12v max). The leakage of these caps varies with the applied voltage, and the 400v capacitors can be completely acceptable in circuits where they only see 12v. Like in DIY radios with battery tubes.

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