Here's a quick pictorial of removing the rusted remains of an original one-touch system and replacing w/ the newer 7443.
Tools used were a big flathead screwdriver, a pocket/folding mini hacksaw and a hammer. (or something to use as a hammer to whack the screwdriver).
In the case of this DE code that was left for free in front of a house, the blades were completely gone. Just the "hub" remained on the inside. On the outside were the rusted thumbscrew/nut and remaining nub of the handle:
The washer doesn't need to be removed. I lifted it to see if it was complete (sometimes they rust and split) and for photo purposes. It's good to leave it on to protect the bowl enamel from the hack saw:
Time to hack off the nub: (wait, what did he say??)
I used the screwdriver and hammer to pop off the nub, and was left with this:
Probably not necessary, but I carefully pried off the washer so that just the nut remained:
Now all you have to do is place the screwdriver on top of the nut and give it a firm whack downward and it should pass through...
...and you're left with an undamaged bowl hole. (It's good to have an undamaged bowl hole).
Follow the instructions for blade assembly. Installation is straightforward and simple. Takes less than 10 mins, even for a dummy like me:
Here's the tool that makes it a snap: