My knowledge is certainly not extensive. I believe that if the reverse is different, as in my case, it is a Trajan reverse, then it is also a mule. I have read different things, some say they were made that way so that the forgers could identify and avoid them, some that they were put together by people who did not know what they were doing. It is an area I would like to gain a bit more knowledge. If you read further up the posts here you will see some links to interesting sites. Yes I love the patina
Certainly many fourrees are correctly matched but it is very, very unusual to find a solid silver mule of official style but nothing special to find a mismatched mule. Most of the solid mules are from mints that off the beaten path rather than Rome.