NOS Shimano chainstay rear derailleur cable stop. These were manufactured in the 1980's for certain 10 speed road bikes.
Material: Stamped steel
Size: Not sure how these are sized - I've never seen anything other than this size and the hole is oblong, not round.
NOTE: This is an NOS item (New-Old Stock), meaning it is brand new, but very old inventory (probably about 25 years old). These came from Shimano in bulk, therefore there is no retail packaging. As always, NOS does not mean perfect - it just means old and unused. If you want new, buy new - NOT NOS.