[Jake Blake]

Isn't there a schematic available online anywhere?

I've put a polarized cord like that on the 48-1256, and I took care to keep the neutral to the side closest to chassis ground. But more and more, I'm developing the mindset ( like many manufacturers), that anything with a metal case ( or chassis) should get a 3-conductor grounding power cord.  I put one on the 89-B years ago. I feel the safety it affords trumps a rigid, original restoration value.  And in the case of the 89B and the 39-6 C I, it nicely eliminates the need to have a separate grounding wire coming out separately,  out the back of the set .