If you look at the Legacy Pro MSDS sheets they are made by PSInc who were/are the manufacturers of Unicolor chemistry as well.
ok then: how can we test whether or not the Legacy product has the secret X?
Maybe find a film that has excessive fog in D-25/23 and then test in both real Microdol-X and then in Legacy?
Microdol-X was brought out because of Dichroic fog issues with a new generation of Kodak films, it's seems to have been a US issue as Kodak kept selling Microdol in the UK for a while. Kodak Ltd made the Kodak films for the UK and Europe and there were differences in Tri-X in particular to the extent taht that the datasheets gave different times for film made in the US, Canada & the UK.
With the modern films I've tried there's no need for a silver sequestering agent. I ran some tests about 5 years ago with a cross between D25 and Microdol/Percetol. Essentially D23 lus Sodium Chloride and a small amount of Potassium Metabisulphite