Hello Harsh,
How did you lost your configuration? Did you selected install configuration NO?
ucs file generated on 12.1.5.x can't be restore on version 12.1.3.x. You need to have ucs file taken when system was on 121.3.x. Here i can suggest one option. If you still have 12.1.5.x installed on the other boot location. Boot F5 from it and select install configuration NO. So when F5 gets booted, system will have all the configuration running on version 12.1.5.x.
Then again you can try to downgrade to target version with selecting install configuration YES.
Hope it works for you!
Mayur