Currently, TWDB serial numbers for Facit 1620s ends in 1973 with a maximal number of 885617.  Yet, in the photo section of TWDB, many owners of Facit 1620s post machines with serial numbers of 6,xxx,xxx and 7,xxx,xxx, all also made in Sweden.  These owners often put 197? or 197x as their machine’s date of production, owing to the lack of a reference point in the TWDB serial number listing.
 
Additionally, repair guides for the Facit 1620 (https://dingasty.com/?p=190) and reviews of the Facit 1620 by trustworthy sources (https://oztypewriter.blogspot.com/2012/08/the-facit-1620-portable-typewriter.html) do not appear to differentiate between different “generations” of *Swedish-made* 1620s.
 
I would like to venture a few observed differences between different generations of 1620s based on my own direct experience with these machines.  I use “generations” loosely, as I have no way of knowing whether these observed differences actually correspond to chronological production changes, as, say, with the earlier curvy Hermes 3000 versus the later boxier Hermes 3000, or the very early Smith Corona 1930s flattops with the shorter return levers and the paper fingers versus the slightly later 1930s flattops with the longer return levers and the paper bars. 

Perhaps other members might also add their own observations, some of which may surely contradict and/or revise the patterns I outline below.
 
“Early” 1620 Generation, generally corresponding to serial numbers that currently exist in the TWDB:
 

• Badging tends to be on the lower front of the ribbon cover, facing outwards towards the keys.  Lettering and numbering on badging tends to be brown, and includes the logo with a smaller circle within a larger circle
• Carriage housing color scheme is bi-color, with both cream and brown
• Platen knobs are shorter, and both right and left knobs attach via set screws
• Variable line spacing is engaged by pulling the left platen knob away from the carriage
• Platen has a metal rod that inserts all the way through it; removal of platen requires removal of this rod [the instructions for platen removal at the website mentioned above correspond to this design]
• Lock nuts for adjusting the escapement trip and the line lock/margin release are 6 millimeters, and the black metal plate they mount to is larger in size than the later generation
• The ribbon color/stencil selector indicator is made up of protruding red, white, and black dots

 
“Later” 1620 Generation, generally corresponding to serial numbers that do not currently exist in the TWDB, e.g., 6,xxx,xxx or 7,xxx, xxx:
 

• Badging tends to be on the top left of the ribbon cover, facing upwards towards the sky.  Lettering and numbering on badging tends to be black, and does not include any circle logo.
• Carriage housing color scheme is mono-color, lacking the bi-color cream accents of the earlier generation
• Platen knobs are longer, and protrude wider from the body of the machine than the early generation.  Both left and right platen knobs attach to the platen by screw thread, and lack any set screws.
• Variable line spacing is engaged by pushing a button inwards on the left platen knob
• Platen does not have any metal rod running through it.  Rather, the variable line spacing is created by the mounting of the platen gear onto three retracting spikes and held in place by a large C-clip [the instructions for platen removal at the website mentioned above would not apply to this generation of 1620]
• Lock nuts for adjusting the escapement trip and the line lock/margin release are 5.5- millimeters, and the black metal plate they mount to is smaller in size than the earlier generation.  However, the actual screws are identical in diameter, and will accept the 6-millimeter locking nut from the early generation.
• The ribbon color/stencil selector indicator is a blue, white, and red sticker with each color occupying a rectangular shape.

 
Aside from these changes, many aspects of the two generations are interchangeable.  For example: the outer shell; the ribbon cover; the part with the bell / line lock / margin trip; most or all of the screws and springs; and even the ribbon color/stencil selector indicator.
 
I invite other members with 1620s to add their observations. 

Additionally, I have not found any information on where the number 1620 derives from.  The leap from TP1 to TP2 to 1620, as opposed to, say, TP3, makes me curious about the significance, if any, of 1620 in Swedish culture or history.  (I have also seen some 1620s with Swedish keyboards badged simply as Facit Privat, with the Privat in cursive.)
 
Cheers!