[HAM] 1955 Hammond M3.

[mike p]

Josh,  a stock M3 as yours sounds to be,  only comes with the main amplifier 
(  the one you refer to as the left one )
A popular option on an M3 was to add reverb . Many reverb amps were mounted 
to the right of the main amp simply because the room exists there .  Some 
reverb speaker cabinets had the amplifier within the cabinet ,  I have seen 
these exist both ways .

And what you observe mounted to the top cover of your M3 is the Vibrato line 
box .   Sounds like you have an earlier M3 like a 1955 as you suggest in 
your title.   Mike