The pre bleach contains Sodium Formaldehyde Bisulfite in an acidic solution. This decomposes into Formaldehyde in the bleach.
There are two other Formaldehyde precursors that could be used as well. We chemists do this to confuse y'all!
Now, Fui-Hunt may have changed the formula of the kit to work more closely with their film now that Kodak has discontinued E6 films, but IDK about that.