I wonder what the mods from 1959 spec are?
It'd have to be something to replicate Randy pushing his heads with an MXR Distortion+.
Apparently it has a "cascade mod" whatever that means.
I have a vague recollection ( from being at college in the 70s ) of 'CascAde' meaning that you stack the triodes in the preamp valves Anode to Anode, as opposed to 'CascOde' - which meant stacking then cathode to cathode.
Whilst I did not learn that in the strict context of guitar amps, I presume that if the basic principle was exploited for the guitar amp, conditions optimum for saturation ( or overdrive ) where reached pretty quickly - using a minimum of additional ciruitry .
I could have a rusty memory though, so I would be willing to be contradicted by one of the current amp techs on here... :)