Differences between 20C & 15?

[Bugface]

I recently became the proud owner a 15.  I can't seem to find much info on these 15's, possibly because they were only made for two years?  
20C's and 15's always seem to be lumped together in discussions.  Why?, what are the differences?
I also noticed that most of the machines discussed here have "letter" models numbers, (eg. AG, OC3, HG).  Does my 15 have a "letter"?  
Thanks
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[Bob]

The differences between the 15 and 20c are mostly in the track frames. A few improvements. Other than that I can't think of any. The model 15 is a 15. No letters for it. The 15 and 20c are very similar to the A series. Maybe a little different gearing but basically the same.