I can't find any reference to this anywhere in this forum. My car is an April 1971 240Z, so I believe the wheel cylinder is the early version. I'm cleaning up the rear brakes and reassembling the wheel cylinders. Inside the wheel cylinder is a spring (part 8 in the FSM diagram below).

The diagram and associated text in the Haynes manual (which includes the same diagram) both indicate that the spring goes into the cylinder small end first. The exact text in the Haynes manual is "Insert the spring (narrow coil first) into the cylinder and then dip the piston in clean brake fluid and enter it into the cylinder. Fit the dust excluder and spring clip.". If this is the case, the spring would be loose inside the cylinder and, it seems to me, could end up turning sideways and getting trapped, rendering the brake useless. Should it not be installed the other way around, with the small end attached to the piston as shown in this photo (this is the one I haven't cleaned up yet) so it doesn't float around?

 

Unfortunately, I didn't take pictures when I disassembled the cylinder, so I don't know how it was originally assembled.

I'm also missing the dust excluder (part #2 in the diagram) for one side of the car. Any idea whether these can be obtained anywhere without buying the entire assembly? The inside of the drum assembly on that side was a real mess because it was missing.

I think you might be correct on the spring.  Have you checked the part number at Courtesy Nissan?