IIRC, there is also Succinaldehyde Bisulfite adduct in this prehardener formula. This compound works like Formaldehyde but leaves the emulsion in a more swollen state. VPooler, your results seem to suggest that you need this compound in your prehardener.
Succinaldehyde was used to lower the amount of formalin put in the solution. Can be substituted with more formalin. I rummaged about in my shelves and found the venerable soviet-era photorecipe book which contained various formulas for hardeners. The formalin/succinaldehyde and the respective neutralizer can and will fog the film if not made properly so to answer my own question - it can't be made much simpler. And from what I heard, it emits a noxious gas when neutralized. I also remember PE from some post in another thread mentioning that he used chromium alum for some of his earlier color work.
Hardening stop bath, however, is made by mixing up the following:
Water (30-45 degrees Celsius) 500ml
Chromium alum 15g
Acetic Acid (28%) 14ml
Cold water to 1 liter
I found from the same book some advice on hardening the emulsion of both soviet and Orwo color materials. A lot of them are basic chromium alum solutions, sometimes with some boric acid or such added, only one contains formalin. So I am pretty confident. The content of chromium alum ranges from 10g to 20g per liter of water and should be discarded once the solution turns greenish. The chrome alum hardening is recommended for tropics, when the temperature reaches 25...30 degrees Celsius and it is claimed safe to use in this range as a pre-hardener. The recipe just calls 10g of chromium alum dissolved in a litre of water for pre-hardener, for using as a stop bath, the recipe is described above. Maybe cold (20 degrees) paper developer for 'bout 10 minutes, followed by hardening stop bath, a good wash and ECN2 at 38 degrees for 6 minutes IF the emulsion can withstand it. If not then 30 degrees and some 10-12 minutes. Might as well try pre-hardening with a strip.
Anyway, I got half a kilo of chrome alum off evilbay for some 20 of my European moneys, let's see how it works. I am excited as the film itself has survived the test of time with grace, I imagine I could rate it even 64ISO or maybe more as the 50ISO results were good, no fogging whatsoever.