[morzh]

From this schematic (and let's face it, not the best print I ever saw, talking of the one on the NA.org) I did not see any DC path from either end of the R507. So unless I miss something your de-soldered value should be the same as in-circuit one. There may be some slow RC there, but still.....although it is indeed interesting why it is then lower than 4.7M and not higher as might be expected with old resistors.