1988 brought the second generation of GLOCK pistol into production, with a few upgrades and new additions to the lineup. Introduced in 1988 were two new production models, the 19 & 17L (Long Slide). 1990 saw the introduction of the first ever GLOCK .40S&w caliber hand guns, the 22 & 23, and also the 20 in 10MM Auto and 21 in .45ACP. The 24 in 1994, which is a version of the 17L in .40S&W. A .380ACP version of the GLOCK 19 in 1995, but was not released in the U.S. due to ATF requirements. A type of serration/checkering was added to front and back straps of the grip, I have also heard it called RTF1 (Rough Texture Finish version 1). There were two versions of the second generation frame. The two pin, which had two pins (one for the trigger and the other for the trigger assembly in the grip) for all 9MMs and the 25. The 3 pin, which was a 2 pin frame with an added pin over the trigger pin, it is for added support of the locking block in larger caliber, higher power pistols, first introduced on the 22 & 23. The two rear frame rail tabs are longer than 1st & later generations. The guide rod was changed to a captured spring assembly sometime between 1988-1990. Magazines were modified to a Full Length Metal Lined/Drop-Free design. Redesigned followers, and better springs and base plates added as well. Model 17s got a thicker walled barrel. New ATF regulations required serial # plates to be added to the dust cover in front of the trigger guard, still using a seven digit serial number.