The term Flighter has come to mean any pen consisting of a steel cap and barrel, but to be accurate, the term applies only to pens made by Parker. The name was first applied to the all steel version of the 51, introduced in 1948. Since then, there have been Flighter versions of almost every model made by Parker, and these all steel pens have proven to be very popular. While most are relatively easy to find, some, such as the 21 Flighter, possibly a prototype, or the 105, an English model made as an economy version of a pen normally offered in a gold fill finish, are much harder to locate. In addition, some Flighter models were made only in certain countries, such as Argentina, making the process of collecting Flighters just that much more challenging.