10-14-2021, 09:40 AM
Could it possibly have been a secondary amp for one or more Wired Remote speakers? The lower knob seems to be the style used to switch between sources (eg - main speaker, remote a, remote b) and the other two controls could have been for on/off and volume. The jewelled "on" lamp mounted above the dial would make perfect sense, letting you know the amp is turned on. The different U speaker and the modified connections between the output transformer and the voice coil would make a tap to take signal to the input of the amp.