@MIKEIRWIN: Like you, I always thought it was a casting. Powdered metal technology was still in its infancy in 1964, and I've been skeptical that a receiver could be made with the necessary tensile and shear strength for that application. However, I'm not a metallurgist.
I've never found an authoritative source for the casting versus sintered metal question in the post '64 receivers. Apparently you did. Thanks for setting the matter to rest.