I have been working with typewriters for 35 years now.  Looking on the forum and on the database has me confused about one thing:  What do they call certain Underwood typewriters?  Oh, I know an Underwood Model 5 from a Touch-Master Five; a model SX-150 from a "Golden Touch," and an Underwood Model 6 from a Master model.  But what I don't understand is the difference between a Rhythm Shift, a Rhythm Touch, and an SX-100.  I know that all these tend to describe Underwoods made between 1946 and 1953, of which I owned many, but I was never able to pin down a designation for these machines.  My best guess is that the Rhythm Shift described the first Underwoods to have the basket shift, beginning in May, 1946.  SX-100 would describe, I think, an Underwood made from late 1948 to late 1953--these were the ones made with the one-piece aluminum-frame carriage with an easy remove platen, and an underslung return lever, painted a taupe color with five ribs in the stainless trim instead of three on the rhythm shift.  But which ones were the Rhythm Touch machines?  Were these the ones made from 1947 to 1948? 

I'm just guessing, but all of these machines were alike in many ways--the latter having a somewhat different carriage structure.  Again, I should know this, but does anyone have any ideas which designation started with which model?