[Mondial]

Looks like capacitor 21 is not a paper cap but actually an electrolytic in a waxed cardboard tube. Part number on the cap agrees with the number and description in the parts list. Cheap electrolytics were actually made this way in the 30's and 40's. This also goes for cap 20 which was also a cardboard electrolytic. 

Parts list specifies both cap 20 and 21 as electrolytics, the confusion is the waxed cardboard case which makes it look like a typical paper cap. Polarity is noted by the black band but I am not sure if this marking indicates positive or negative. It looks like the end of cap 21 connected to the 6U7 pin 6 connects to the IF transformer primaries and a film bypass cap, which should be the negative of the electrolytic. Positive of cap 21 should connect to the B+ supply line and negative to the connections to the IF transformer primaries.

The wiring of the cap 21 to the 6U7 socket is also confusing, but it appears that the tube pin is not used on the 6U7 and the socket terminal is only being used as tie point for various wires. If you trace the circuit connections of cap 21, they should connect across the primary winding of saturable transformer 4 which is part of the tuning indicator circuit. The brightness of bulb 33 varies with tuning as the plate current of the 6L7 and 6U7 changes with AVC control voltage.