Clip and Pin
For this type, the lower part of the spool has a smaller diameter than the upper part and a chrome plated clip is attached to the lower part. A pin (screw head) is provided to engage one of the film sprocket holes. Serrated upper and lower flanges allow the spool to be rotated into place, moving in the direction indicated by the arrow. When loading this type of take-up spool, the film end is slipped between the clip and the spool, then film is advanced while ensuring the pin has engaged a sprocket hole.

Single Slot
For this version a single slot on the spool has replaced the clip and pin. A small tab near the lower end of the slot now engages the film sprocket hole. The serrated flanges and direction arrow are still needed to rotate the spool into loading position. Loading this type of spool consists of inserting the film end into the slot, then making sure the tab grabs a sprocket hole as the film is advanced.

It seems that the clip and pin type spool and the single slot spool were used interchangeably in the early cameras. I have not been able to correlate a spool type with the camera serial number in a rational way. None of the instruction books I own show the clip and pin type.  I have one camera with a serial number in the 4000s, and one with a serial number in the 12000s and both have the clip and pin take-up spool. And yet, there is a another body with a serial number in the 6000s with a single slot type. Maybe the upper deck housing with the serial number was changed when a body was repaired? I am open to suggestions.

Multi Slot
This is the final version. The upper flange is gone, along with the serrations on the lower flange. Around the circumference of the spool are five slots, and obviously some inner mechanism to grab the film end during film advance and yet release it for film rewind. During loading, the film end is merely slipped into an accessible slot and film is advanced, while watching to ensure it is secured. With five slots, one will always be in position so there is no need to rotate the spool.

I like to use a polarizing filter at times, especially to darken the sky. And since shots with wide angle lenses usually includes a lot of sky I missed this ability with one of my favorite lenses, the RE Auto Topcor 25mm f/3.5. Because of the optical design, a normal 62mm filter will cause vignetting so Topcon made sure no filter is attached by the simple means of not providing filter threads. One day I found a Series IX polarizer on a 'junk' table in a camera store and tried it out, mounted in this lens's strange hood. But each adjustment of the filter requires rotating the front part of the hood to line up the rectangular opening with the camera body. Not a good arrangement for fast shooting.

Later, I came by a spare hood for this lens, again from a 'junk' table, and having access to a metal lath I cut out the rectangular opening on the front. The images below show the result. Now I am able to adjust the filter and shoot, without taking my eye away from the eyepiece.