Oh, the grip on the paper is better than a hard platen!

Oh!  I see.  You used them as backing sheets.  I plan to remove the platen rubber all together and rewrap the the platen with the rubber.  A homemade platen replacement.  It's hard for me to put $100 into a platen replacement on a typewriter that I paid $0 to $50 for.  I'm a lot less squeamish about modifying typewriters than many people, and I enjoy the process. If it seems like the rubber wrap works on the "throw away" typewriter, then I'll do it good machines like my Adlers and Triumph.  The only machine that I have with a reasonably soft platen is an Adler Tippa 1 that is 93A, and it's obviously softer and quieter.  All the others are 98 to 99.5.  My durometer only reads in half unit increments, and even on glass it reads 99.5.  I was taken aback by what a difference it makes to go from 99.5 to 86 to 89.  It's so much more pleasant from a noise, print quality and touch perspective.  It does seem like it somehow changed the feel of the machine.  Maybe a little snappier return?  Maybe I'm making that up.

I see that some others have tried PVC tubing as a platen cover, but they didn't include hardness readings.  I purchased an inexpensive digital Shore A durometer from Amazon.  The platen on the SMC Sterling was 99.5 (I accidentally wrote the hardness of a different platen on the typing sample picture, and 98 is incorrect).  I removed the platen material and replaced it with $3 worth of hardware store reinforced PVC with a 1" internal diameter.  I used a heat gun to warm the PVC to slip in onto the platen core.  It measures 80 to 83 Shore A.  With two sheets of paper, it reads 86, and with Scotch single sided laminate it reads 89.  It was interesting to see the similarity between the imprint of the durometer pin and the period.  It is MUCH quieter, and it also has a much better imprint quality.  I also like the aesthetics of the reinforced PVC.  I used reinforced PVC because it was harder than non-reinforced PVC tubing. 

I tried covering the platen with Fel-Pro 3019 Gasket material because I had some that I used to line the jaws of my woodworking vise.  It was too soft, 69 to 73, and the letters embossed the paper. 

My next experiment will be with some 90 durometer neoprene rubber sheet.  I didn't find a source for 90 durometer tubing, so that's why I'll try the sheet and cement it on. 

Pete, I found it was easy enough to change the touch on my Perfekt De Jur with some fiddling.  I don't know what your background or comfort level is with making modifications, so maybe you've already considered this.  I do a lot more monkeying than my knowledge or skill warrants, but I enjoy learning.  I wanted to make the touch lighter, so I made a little link out of a staple and inserted the new link at the top of the universal bar return spring.  The spring attaches under the right ribbon spool.  It was easier to do with the body off the machine. If the link is too long then the ribbon vibrator won't drop all the way because it has a mechanism that works opposite the universal bar.

Unfortunately, the bar that holds all the springs between the key levers and type bars isn't adjustable like it is on some machines.  I plan to add some length to those springs or modify the machine to allow adjustment at some point to see what difference it makes in the feel of the machine.  I really like how it types, but I have a hard time not wanting to modify and adjust things even if I ultimately return to the factory settings. 
 

Good to know.  The O-rings must change the gear ratio some too since they are a different width than the belts.