Tetenal C-41 Stabilizer contains Hexamethylenetetramine (hexamine) which is converted to formaldehyde in the solution. The second component is 1,2-benzisothiazol-3(2H)-one, an industrial biocide.
Since popular biocides in their pure form are sufficiently toxic (also to humans) to make them unavailable to amateurs like me, Formalin is the best we can do if we want to self mix. The likely reason, why Tetenal still has Formalin in their final rinse is the fact, that they sell one and the same product for C-41 and E-6.
I believe it was also used as a hardener. Here's my favorite formula: formalin 120 ml; vodka to 1L.
It's 120ml Formalin in 2l bock beer in Central Europe
