I have two 64-8's, a 4" and a 3". The 3" was returned from Europe, lightly used, and the 4" appears to be unissued even though it was advertised as a Security/LEO return.
Funny thing about the 3", it chambers .357 ammo. I haven't shot any yet, and only noticed when I cleaned it and a 65 after a trip to the range. I load all my guns before putting them back in the safe, since there is nothing more useless than an unloaded firearm (and I am not going to argue this point, so don't bother).
The 4" 64 won't chamber 357, but the 3" does. I am guessing that one of two things happened. Either it was shipped with a 65 cylinder as part of the original contract, or someone reamed it later.
S&W did make a batch of 64-1's chambered in 357 Magnum, but with the introduction of the Model 65, those 64-1's are likely the only factory made 357 versions.