…do not exist if you are speaking of those made by Fabrique Nationale, the original manufacturer of the design. This holds true whether the pistols are sold under either the FN or Browning logo. None of them are stainless…not a single one.
That does not mean that some may not have appeared to have been of stainless-steel construction. Some Hi Powers imported by Browning Arms have sported both electroless-nickle or hard chrome finishes, either of which may be confused with stainless. Electroless-nickle tends to have a slightly yellowish hue to it while hard-chrome does not. It is either white-colored or sometimes appears a very pale light blue.
Several years ago, a company called Baford Arms did produce stainless steel Hi Power style pistols but they have long since gone out of business and there is currently a company in Florida producing both slides and frames of stainless construction. Some aftermarket Hi Power parts have been manufactured in stainless but FN has not manufactured any stainless-steel Hi Powers.
On some auction sites, FN and/or Browning-marked Hi Powers are being advertised as being of stainless steel construction when they simply are not.
Such Hi Powers can indeed be very fine handguns and either electroless-nickle or hard chrome does enhance corrosion-resistance but the pistols are not stainless steel.
Best.