Your trigger bar has 4256 marked on the cruciform portion that sits in the trigger housing? That trigger bar was known to cause light strikes with certain primers. Glock updated the trigger bar for the G21 to the 4256-1 trigger bar which has a longer kickup sear which pulls the firing pin back further before releasing it so that light strikes won't occur with most primers.
http://glocktalk.com/forums/showthread.php?t=1335082
^^^The 4256-1- is a modified version of the 4256-1. The 4256-1 is for G21's, the 4256-1- is for G30's. The kickup sear on both is longer than on the 4256.
But I don't think that would explain the primer strikes being off-center. According to one of the Armorer's Manuals, some possible causes for Off-Center Light Strikes are:
1. Tight extractor(either out of spec or it has plenty of gunk built up on it, along with gunk built up in the slide around the extractor)
2. Dirty gun(putting the slide slightly out of battery, some primary causes would be a dirty chamber or an out of spec round)
3. Slide lock reversed or not bevelled
4. Weak recoil spring(not holding slide in battery, seeing that your G21 is an early model this may be the case if the recoil spring has never been replaced)
Also the early Gen2 G21's were made with a 90 degree extractor which is no longer sold. Later Gen2 G21's had a 15 degree extractor, which is still sold. If your G21 has the 90 degree extractor and you want to replace it, you'll have to send it to Glock. I don't know if they still do this, but they used to cut G21 90 degree slides to 15 degrees and refinish, install the 15 degree extractor, then send it back to you. I'd detail strip clean the slide particularly the extractor and the part of the slide it fits into, and replace the recoil spring before I'd consider sending it to Glock. I'd also test it with factory ammo. If this problem occurs with reloads but not factory ammo then the cause is the reloads being out of spec.
