Some more mail locks have been sold on eBay and the prices are all over the place.
This lock sold for $21.50 (Plus $5 shipping). Nice wear but no key:
This other lock also has nice wear and more importantly, what appears to be an original key, which I have not seen before:
BRASS STREET LETTER BOX LOCK!!!! 3". Vintage Brass U.S. Mail Padlock in working condition. On the front is STREET LETTER BOX LOCK,UNITED STATES 12. On the key is U.S.Mail and the no. on the back. - eBay
This lock also has the number twelve on it which seems to be a rarer number for this type lock. A least until eBay is buried with more locks of this number type.
This lock sold for $127.51 (Plus $4.80 shipping). Was the high price a result of the number of the lock, the key, or both?