The default management key (9B) on YubiKeys with firmware up to version 5.7 is a 3DES key with value 010203040506070801020304050607080102030405060708.
For YubiKeys with firmware version 5.7 and later, the default management key uses AES-192 instead of 3DES, The management key uses the same default value (3DES and AES-192 keys are the same length).
From yubico's docs:
The MgmKey assumes 3DES for all cases currently.