In retrospect, the opening post probably gave an impression that I'm overly interested in the monetary value, that's not the case. I was initially puzzled by the difference in original costs between the 520A and the Mullard MTS501. Then realised that the corresponding Mullard is truly the MTS5
21.
More than anything I'd love to, some day, see sets such as this go to a good home (better than mine, with attention from expert restorers). I'm aware of
Sets, Parts and Service Information Offered, but that's not why I joined; for now it's far more enjoyable to read people's stories and opinions.
Thanks again