A licence issued under 61.75 is a perfectly valid licence in its own right. Where a 61.75 licence holder passes a US IR Check Ride and completes the US IR written, the 61.75 will be endorsed with the phrase "INSTRUMENT AIRPLANE U.S. TEST PASSED".
Where the 61.75 licence holder has an ICAO instrument rating on his underlying licence, he can also validate that onto his he US licence by passing a cut-down version of the US IR written aimed at foreign pilots (known as the IFP). Whilst there is a little inconsistency on licence endorsements, the standard licence endorsement goes along the lines INSTRUMENT FOREIGN PILOT TEST PASSED