would be surprised if there was such an adapter as WTF would it be useful for?
If you have a multimeter you can measure the value of the two resistors and hence calculate the attenuation.
I believe (as I've never had a set in my hands) that its a potential divider something like this:-
input--- R1---output---R2---common (gnd)
Measure the values of the two resistors (input to output/output to common) and the attenuation will be 20Log(R2/(R1+R2))dB
Cheers,
DV
I have a spare pair, minus 10db, if anyone's interested, £22 and they're yours.