An Olivetti ICO is a bit special, a good reason to use the new old Olivetti ribbon I picked up years ago.
The ICO (or MP1) will not take a full 10 meters, only about 6 meters or less. It has the small spools, like e.g. the Remington Portable machines of the era. So a length of the silk ribbon was wound onto the original ICO's spools. (The Olivetti MP1 is overall remarkably similar to a Remington Portable 3.)
Replacing the ribbon is fairly straightforward, no fiddly or fragile reversing mechanisms. The spool-covers are held on by spring-clips - pull up a little and they can be slid off sideways. Replacing them is similar, probably best sliding them on whilst pressing down at the same time. Simply pressing down will not work so well - may partly account for some of the machines missing their spool covers.
The ribbon is fainter than it will have been when new - even inside the shrink-wrap it dried out over the decades.
Nevertheless, the Olivetti MP1 now has very crisp and readable Olivetti typing!