I have seen that booster on drumbrake cars, haven't seen it on a disc car (but I will admit I haven't seen everything!).

That part#, according to this auction and Rock Auto, fits 65-up cars, yet discs weren't officially available until 66 (I heard some copcars might've gotten Kelsey discs in 65). I'll go on a limb and say that is a drum/drum booster. Regardless, it's only $129 at Rock Auto.

Look at 54-73700 and 54-73600 at RockAuto. The -700 is the 'large flat one' I mentioned earlier (fits 71-73), and the -600 is the 69-70 disc style (looks much like the fabled 'Hemi booster') and is pretty much unavailable.

Check the Drydock too, there are a bunch of disc booster references there (but you'll hafta dig for them).