If the back door on the Castle Front is original than it will look like the one in the below photo and the lock will only show the key opening which would look like the one circled within the below photo!
If the key entry on the lock is like this one than it is original so having a locksmith pick it or even try would be unlikely if it is an original lock which has not been re-keyed at one time as sometimes in re-keying an original Mills lock some tumblers are removed making it easier to pick!
If your lock is a later replacement style as commonly used picking it would be easy, or should be!
Oh and so you will know depending on the serial number the Mill's Blue Front called "Castle Front" by route operators is probably mid 30's!
Bona Fortuna!
MONTI