I think the only option readily available for UL-C is a brute force attack, which both the flipper and PM3 support
I don’t think you can sniff keys for it
@equipter has some kind of ancient tome that lets him generate the keys at least in some cases, and he may be able to help you more
In the meantime you may try:
hf mfu cchk -f mfulc_default_keys.dic
BUT:
Even if you find the key, it sounds like it may not work for you 