Actually, YES. If you scrolled further down this page you would see the thread where I found, researched & ultimately purchased a US marked S&W M56 last week. Between these 2 threads, you can see the different details.
Sighting @ LGS: Military Model 56 w/ US marking
There are about known 50 survivors of 15,205 made, in the database. SN range is random between K500001 - K250800. They were made 1962-3.
(PM sent to dmc)