Unfortunatly, I don’t remember the make and model number of that specific radio but while scanning the RCC manuals (from beginning to 1980) in high definition I came across the schematic of a series/parallel filament arrangement. It was an 8 tube radio from the late 20’s or early 30’s. All tubes were 2 volts, 7 had the same current value and the output tube was twice the current value of the others. So two groups of three tubes in parallel and one group with the output tube and the remaining one in parallel, the three groups in series for a 6 volt storage battery supply. Not necessarily the lowest consumption but yes, series/parallel were used.