A Primer on Typewriter Ribbons & Spools by [[Damon DiMarco]]

Typewriter ribbon is 1/2" wide (12.7mm) which has been standardized as DIN2103.

The DIN 32755 spool, aka a Universal spool, or a Group 1 spool.

Olivetti typewriters use Group 4 spools.

via u/guneeyoufix at https://new.reddit.com/r/typewriters/comments/1amo13f/cheap_ribbons_for_uk_typists/

https://www.mrmrsvintagetypewriters.com/en-us/blogs/news/which-typewriter-ribbon-do-i-need-for-my-typewriter

Lists of the primary spool types and typewriters which fit them.

The three most common typewriter spools:

• GR1 (Group 1)

  • Used by most German machines.
  • Sometimes known as DIN 2103 or 32 755.
  • 50-54mm Diameter.
  • 5mm center hole.
  • Usually 1, 3, 4, or 6 round drive holes.

• GR4 (Group 4)

  • Used by Olivetti and some other Italian machines.
  • Early models require a separate spool nut to fasten.
  • 50-52mm Diameter.
  • 6mm center hole.
  • 3 rectangular or half-moon drive holes.

• GR9 (Group 9)

  • Used by most Japanese machines after 1970, Hermes, some portable models from Royal and Underwood, and Olympias made for the US market.
  • 51mm / 2" Diameter.
  • 4mm center hole.
  • Usually 1, 4, 6 or 8 round drive holes.

via u/Koponewt