This is an old firmware that does contain some lag and bugs. However it is good if "the things with your phone go wrong and you have to revive it". The flex is GSZSMTRT676NS01D and the langpack is 0615.
How to flash:
1. Turn off your phone. Hold the Voice Key and press the Red Key to go into bootloader mode
2. Start RSD Lite and select the extracted 29p sbf file.
3. Click start and wait until it finishes
4. At the end the program may show "Flash Failed" - no worries, just unplug the phone, remove the battery and plug it back.
5. Start your phone and enjoy 29p