I realise I may be going up a blind alley here...but, as you quote, the US 'restricted' license is subject to the same limitations and restrictions as the foreign (UK) license.
It does not say that it is subject to the same ratings and privileges and I am led to believe that those are differently things.
As I understand it, the FAA licensing system (unlike the EASA system) no longer has type ratings for helicopters under a gross takeoff weight of over 12,500 pounds.
Therefore a full FAA license covers flying any helicopter that meets that requirement (except Robinsons, but that's another issue).
The question is whether the 61.75 piggyback FAA certificate follows that same model.