Welcome to the forums. Your Competition is a European market model manufactured in 1980. An annual production of 100,000+ frames is not that large. Some of the larger Japanese companies, such as Bridgestone and Miyata were approaching one million bicycles per year around that era. Unfortunately, we can't tell exactly when during 1980 your frame was built, but based on the high number it may be 1981 model manufactured in late 1980.

Typically, the individual gears on a freewheel or cassette are called cogs or sprockets. What you have, based on the part number, is a Shimano 6 speed freewheel, not a cassette. In a freewheel, the cogs and freewheel body are removed from the hub as one unit. In a cassette, the cogs are removed from the freewheel body, which remains attached to the hub. The combined hub and freewheel body is typically called a freehub.

Yours is a relatively modern, Hyperglide compatible, replacement for the original freewheel. Hyperglide refers the the ramps stamped into the cogs that improve shifting, especially under load. The black large cog is not a replacement. The different colour is simply a relatively modern trend on Shimano freewheels.