I think I've figured it out. I have some documentation from 1972 which lists, amongst other things, which reel uses which spool. The entry for the standard says "no spool, part of reel".

Line was wound on the reels because so many people were putting it on backwards and then returning the reels as defective. Look at the bare body, second from the right and ask yourself this question:
How do you wind the line on this reel.
All the other Zebco reels have removeable spools which means you can attach it to an electric motor and wind the line on that way. This is impossible with the standard's design. The 11 and 22 have the removeable spools.
Note in the other reels the variety of spinnerhead designs. The one on the far right is my salesman's sample. I believe the spinnerhead is made of bakelight. I'm not as sure about the red one on the far left, but note how it is attached with a screw and spring arrangement instead of a nut. Second from the left is an attempt to make the part lighter and out of metal. It scrapes the front of the spool. In the center is a spinnerhead in metal with four pickup points which doesn't scrape the spool.
I now believe the reel was discontinued because the line had to be wound on by hand. Other documentation says that the reel continued to be taken to the trade shows until 1956, I presume to show ongoing product development.
By the way, the reel on the far left is my earliest 33 prototype and probably dates to about 1952 (or maybe a little earlier).
Regards,
Clinton