Actually the 5.5" Extendable Front Sight (or EFS) barrel was released in 1965 and made for about 10 years. You could buy the gun with the EFS barrel or order the EFS barrel separately. Guns with the EFS barrel tend to be worth more than the regular 5.5" barrel, especially if there is a properly marked box or factory letter to prove it came that way. Of the four original barrel lengths; 7 3/8" w/ Muzzle Brake, 5" Field, 5.5" Heavy and 5.5" EFS, the 5.5" EFS barrel is probably second only to the 5" Field (I'm referring to the orignal Field barrels which were factory cut down 7 3/8" barrels) in value and rarity.