Tektronix was famous for their modular scope designs. The scope itself was just the screen unit and the power supply. Everything else was handled by plugins. The 7603 can take two vertical and one horizontal plugin. If these are input amplifiers or a time base is up to the user

It could also be combined plugins, like this TDR/Sampler (7S12) or spectrum analyzer (7L12)

I showed the spectrum analyzer plugin here before. It's this channel's icon.

With the original ~50 year old filter caps

Edit: the measured ripple is always an upper limit

Option 4 is the 'Maximum Brightness CRT' and Option 78 'P11 Phosphor'

No wonder this thing messes with photo sensors