I have a Mallory HyFire 6AL ignition box and an Autometer Super Comp 3904 tachometer. When connecting the tach input wire to the ignition box tach output wire, the tach does not work so it seems that I need a tach adaptor.

Well the magnetic pickup tach adaptor that Mallory sells is about $115 and I have a MSD magnetic pickup tach adaptor (#8920) already in my possession.

Will I be able to use this MSD adaptor with the Mallory ignition box? Looking at the wiring diagrams for both they seem to be wired similar...

Worked for me.
Installed a Hyfire and my `71 stock-tach did not work. Found an MSD tachadapter, tried it and it made the tach work again. At least a 1000rpm off,like before, but looks good working
Both, the Mallory and the MSD are putting out a square-signal, but some tachs need a voltage spike.
The tachadapter just converts the square-signal into a voltage spike.
My Autometer-tach worked without the tachadapter instantly

Sorry for my english, Greetz Arnd