[morzh]

Well, I de-soldered this capacitor, both ends (it's a dual one) and measured it: the leakage is nothing to speak of (on the order of 10 MegaOhms) and the capacitances are 0.35uF and 0.45uF, the latter being the one I was worried about.
The sch. shows 0.25uF and I don't think it matters here, if it is alrger - it is a filter; the more the better (to a point, of course) .

Then I measured the plate voltage of the detector tube while removing and touching the cap wire to the screen grid. No effect whatsoever. It stays 109V with or without the cap.

PS> I did try the tube swap (there are three 24 tubes here). Also no difference.
PPS> While measuring plate voltage (RF screen installed - no good reception) I touched the plate of the 1-st RF 24 tube with the meter, and the reception just rocked me! The sound was loud, clear, well-regulated by the Volume regulator - really great, no noise, almost no distortions. I just wonder if my Detector's plate voltage actually is OK. While in this good reception, I measured it's plate voltage - same 109V, not even close to 30V, like the chart says.
PPPS> The Cathode voltage at the detector is 1.4V (supposed to be 8V). At 8V the cathode current would be 8V/50K=0.16mA, so the drop on 600K of the plate load would be 0.16mA*600K=96V, so 136V-96V=40V which is way close to 30V than 109V is.