A 1925 Marconiphone 'rejectostat' can be seen in a picture half way through this thread
https://www.vintage-radio.net/forum/...d.php?t=146641
As described by Al, it is simply a tuned circuit with a few taps on the coil to cover multiple ranges, the whole thing enclosed in a metal case. Can't say if it works because I still haven't finished the plugin coils (got as far as getting the primaries working and tuning in the correct range).