The user manual (quoted below and linked left) seems to indicate that there was no compliance testing to see how a magnetic field would affect the hardware and potentially, your signal.
"Caution: There are no product safety, electromagnetic compatibility (EMC), or CE marking compliance claims made for the USB-622x/625x/628x OEM devices. Conformity to any and all compliance requirements rests with the end product supplier."
However, the encased non-OEM 6255 has minimal immunity to EMC, according to its specifications manual. The OEM board probably wasn't tested because the purpose of the OEM is to integrate the board into applications, where it could potentially be exposed to a wide variety of environmental factors. Thus, they leave it to the customer to test to make sure everything is functional within their system.
Hope this helps!
Cindy
