If it's a slip yoke trans, it pretty much has to be for a '65 model year car. Curious why you think it might not be??...... The cast pieces; case, tail stock, valve body halves, etc, will have the circular type date code indicating the month, and '64 or'65 inside the inner circle, if indeed this is a factory stock trans. HOWEVER, if im reading the date code correctly (which I may not be, as some early stuff was different)..... I got 1886 for the 10,000 day code, which is the 340th trans built on 26th of Sept, 1966. If this is true, maybe a warrantee trans???
BTW, pretty sure that trans would be very commonly used with STREET WEDGE engines, although sure if thats not the only application.