I can't find this either, I think these are the same as the ones that come with the ONS15454 OPT-PRE/OPT-AMP cards and I have looked at some of the spare ones I have here and the part number appears to be CIS1009.
I think they are supplied to CISCO by Advanced Interconnect.
On the ones I have here there is another reference CISCO PN: 39-0234-01 ADP.OPT.LC.LOOPBACK.ATT 5dB.
Found this link on Google not sure if its any use:
http://www.brokerbin.com/resellers/network/CISCO/CIS1009.html
You don't actually need the loopback attenuators you could just use a patchcord with 5dB loss instead.
Hope this helps