Well thanks to a local FB Marketplace trip with my wife to pick up a vintage sewing machine, I was able to snag another O-U 21 machine & case for $ 15 USD.  I spotted it in a dark corner of the seller's garage.  I knew what it was by just looking at the case and she said " ...it's yours for $ 15... ".

It is as nice as my first machine and I am working my way through the "To Do" list of some items needing done and sorted out.

Nice! Does anyone have an idea of just how many 21’s were sold over the years? Based on the activity of my local Craigslist, I’m under the impression that this must of been one of their very best sellers. There always seems to be one or more listed for sale whenever I check. Not always at bargain prices however, but were this machine to prove to be a good fit for me, I feel that I would at least be able to have my pick of one in great condition.

M. Höhne

Yes, both of my machines have the cross-link you mentioned.  No cross link on my machines on the opposite side...so maybe something running along the left side was cause for the cross link ?  p.s.  I usually do not publicly display SN's for any of my collectible items.
 

Greetings All and a very Merry Christmas

Just a couple of theories to put forward about the 21's. Looking at my Underwood 21 S/N 917777 made in Don Mills Ontario Canada, it too has the E and X links crossed.

Theory # 1: With the letters E and D being neighbors in the basket, is it possible that as typists were getting faster, the E and D type bars were frequently jamming on words ending with ED as in "she talked too much". By crossing the links, the much lesser used letter X worked as a spacer between E and D thus reducing type bar jams.

Theory # 2: Olivetti/Underwood 21's are showing up more frequently simply due to the age demographic of the population. These machines were being bought new by young people going into college back in the early to mid 1960's. Those people are now going into retirement homes and their houses are being cleared out by their children or grandchildren, hence these machines showing up more frequently on local online buy and sell sites.

Any thoughts or opinions welcome as always. All the very best,

Sky

Well the patient (Olivetti-Underwood 21) survived the morning srugery and is doing well.

I tipped the machine on to its side and sprayed the platen knob, release depressing knob, and the ratchet mechanism with PB Blaster 3-4 times over a 3 hour period. 

Slight tap on the release depressing-knob with a plastic block and my rubber mallet did the trick.  Depressing-knob now pushes in and comes out.  Ratchet free-wheels then when depressed.. And the CR lever now ratchets for line spacing.  No need to bend/form any metal and no typewriter was harmed in the process. 

Spent another hours with liberal amounts of denatured alcohol to vacate the smell of PB Blaster from the typewriter. 

Great way to spend a quiet Xmas morning before the dinner activities kick in.  Happy Holidays !!!