If the 1973 Lead & Bass Combo is as rare as hens' teeth, then the 1973 Master Reverb 18w Combo is the proverbial rocking horse shit.
I can't find any other pictures anywhere on the web. From the looks of it, it's lost the tremolo channel and it's using the valve in the V1 position for the reverb drive / recovery. (According to Dr Tube it's an ECL86.)
Intriguing. With just a couple more pictures, I could almost figure out how it's all wired together.